Docs/add markdown page (#3111)

* First draft of add markdown pages

* Polish the draft

* Make grammatical corrections

* Update adding-markdown-pages.md

* Format markdown

docs/docs/adding-markdown-pages.md

@@ -2,7 +2,179 @@
 title: Adding Markdown Pages
 ---
 
-This is a stub. Help our community expand it.
+Gatsby can use markdown files to create pages in your site.
+You add plugins to read and understand folders with markdown files and from them create pages automatically.
 
-Please use the [Gatsby Style Guide](/docs/gatsby-style-guide/) to ensure your
-pull request gets accepted.
+Here's the steps Gatsby follows for making this happen.
+
+1. Read files into Gatsby from the filesystem
+2. Transform markdown to HTML and frontmatter to data
+3. Create a page component for the markdown files
+4. Programmatically create pages using Gatsby's node.js `createPage` API
+
+### Read files into Gatsby from the filesystem - `gatsby-source-filesystem`
+
+Use the plugin [`gatsby-source-filesystem`](/packages/gatsby-source-filesystem/#gatsby-source-filesystem) to read files.
+
+#### Install
+
+`npm i --save gatsby-source-filesystem`
+
+Now open `gatsby-config.js` to add this plugin to the plugin array.
+
+To add a plugin, add either a string (the plugin name) or to pass options, an object.
+For `gatsby-source-filesystem` we pass an object so we can set the file system path:
+
+```javascript
+plugins: [
+  {
+    resolve: `gatsby-source-filesystem`,
+    options: {
+      path: `${__dirname}/path/to/markdown/files`,
+      name: "markdown-pages",
+    },
+  },
+];
+```
+
+Now that we've "sourced" the markdown files from the filesystem, we can now "transform" the markdown to HTML and the YAML frontmatter to JSON.
+
+### Transforming markdown — `gatsby-transformer-remark`
+
+We'll use the plugin [`gatsby-transformer-remark`](/packages/gatsby-transformer-remark/) to recognise files which are markdown and read its content. It will convert the frontmatter metadata part of your markdown file as `frontmatter` and the content part as HTML.
+
+`npm i --save gatsby-transformer-remark`
+
+Add this to `gatsby-config.js` after the previously added `gatsby-source-filesystem`.
+
+```javascript
+plugins: [
+  {
+    resolve: `gatsby-source-filesystem`,
+    options: {
+      path: `${__dirname}/path/to/markdown/files`,
+      name: "markdown-pages",
+    },
+  },
+  `gatsby-transformer-remark`,
+];
+```
+
+#### Note on creating markdown files.
+
+When you create a Markdown file, at the top of the file, add the block below. You can have different key value pairs that are relevant to your website. This block will be parsed by `gatsby-transformer-remark` as `frontmatter`. The GraphQL API will provide this data in our React components.
+
+```
+---
+path: "/blog/my-first-post"
+date: "2017-11-07"
+title: My first blog post"
+---
+```
+
+### Create a page template for the markdown data.
+
+Create a folder in the `/src` directory of your Gatsby application called `templates`.
+Now create a `blogTemplate.js` inside it with the following content.
+
+```jsx
+import React from "react";
+
+export default function Template({
+  data, // this prop will be injected by the GraphQL query below.
+}) {
+  const { markdownRemark } = data; // data.markdownRemark holds our post data
+  const { frontMatter, html } = markdownRemark;
+  return (
+    <div className="blog-post-container">
+      <div className="blog-post">
+        <h1>{frontmatter.title}</h1>
+        <h2>{frontmatter.date}</h2>
+        <div
+          className="blog-post-content"
+          dangerouslySetInnerHTML={{ __html: html }}
+        />
+      </div>
+    </div>
+  );
+}
+
+export const pageQuery = graphql`
+  query BlogPostByPath($path: String!) {
+    markdownRemark(frontmatter: { path: { eq: $path } }) {
+      html
+      frontmatter {
+        date(formatString: "MMMM DD, YYYY")
+        path
+        title
+      }
+    }
+  }
+`;
+```
+
+Two things are important in the file above.
+
+1. A GraphQL query is made in the second half of the file to get the Markdown data. Gatsby has automagically given you all the Markdown metadata and HTML in this query's result.
+   **Note: To learn more about GraphQL, consider this [excellent resource](https://www.howtographql.com/)**
+2. The result of the query is injected by Gatsby into the `Template` component as `data`. `markdownRemark` is the property that we find has all the details of the Markdown file. We can use that to construct a template for our blogpost view. Since it's a React component, you could style it with any of the recommended styling systems in Gatsby.
+
+### Create static pages using Gatsby's Node API.
+
+Gatsby exposes a powerful Node.js API, which allows for functionality such as creating dynamic pages. This API is available in the `gatsby-node.js` file in the root directory of your project, at the same level as `gatsby-config.js`. Each export found in this file will be run by Gatsby, as detailed in its [Node API specification](/docs/node-apis/). However, we only care about one particular API in this instance, `createPages`.
+
+Gatsby calls the `createPages` API (if present) at build time with injected parameters, `boundActionCreators` and `graphql`. Use the `graphql` to query Markdown file data as below. Next use `createPage` action creator to create a page for each of the Markdown files using the `blogTemplate.js` we created in the previous step.
+
+```
+const path = require('path');
+
+exports.createPages = ({ boundActionCreators, graphql }) => {
+  const { createPage } = boundActionCreators;
+
+  const blogPostTemplate = path.resolve(`src/templates/blogTemplate.js`);
+
+  return graphql(`{
+    allMarkdownRemark(
+      sort: { order: DESC, fields: [frontmatter___date] }
+      limit: 1000
+    ) {
+      edges {
+        node {
+          excerpt(pruneLength: 250)
+          html
+          id
+          frontmatter {
+            date
+            path
+            title
+          }
+        }
+      }
+    }
+  }`)
+    .then(result => {
+      if (result.errors) {
+        return Promise.reject(result.errors);
+      }
+
+      result.data.allMarkdownRemark.edges
+        .forEach(({ node }) => {
+          createPage({
+            path: node.frontmatter.path,
+            component: blogPostTemplate,
+            context: {} // additional data can be passed via context
+          });
+        });
+    });
+}
+```
+
+This should get you started on some basic markdown power in your Gatsby site. You can further customise the `frontmatter` and the template file to get desired effects!
+
+## Other tutorials
+
+Check out tutorials listed on the [Awesome Gatsby](/docs/awesome-gatsby/#gatsby-tutorials) page for more looks at building Gatsby sites with markdown.
+
+## Gatsby markdown starters
+
+There are a number of [Gatsby starters](/docs/gatsby-starters/) that come preconfigured to work with markdown.