Update Authentication doc to cover all backends

docs/authentication-backends.md

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+# Authentication & Backends
+
+Netlify CMS stores content in your GitHub repository. (GitLab and Bitbucket coming soon!) In order for this to work, you need to authenticate with GitHub, and that requires a server. We have a few options for handling this.
+
+## Git Gateway with Netlify Identity
+
+[Git Gateway](https://github.com/netlify/git-gateway) is a Netlify open source project that allows you to add editors to your site CMS without giving them direct push access to your GitHub repository. [Netlify Identity](https://www.netlify.com/docs/identity/) service handles the authentication and provides a simple interface for user management. The Netlify CMS [Test Drive](/test-drive/) is a working example of this backend.
+
+To use it in your own project, follow these steps:
+
+1. Head over to the [Netlify Identity docs](https://www.netlify.com/docs/identity) and follow the
+   steps to get started.
+2. Add the following lines to your `config.yml` file:
+
+    ``` yaml
+    backend:
+      name: git-gateway
+      accept_roles: #optional - accepts all users if left out
+        - admin
+        - editor
+
+    ```
+
+3. Optionally, you can assign roles to users in your Netlify dashboard, and then limit which
+   roles can access the CMS by defining the `accept_roles` field in the `config.yml` example above.
+   Otherwise `accept_roles` can be left out, and all Netlify Identity users on your site will have access.
+
+## Git Gateway without Netlify
+
+You can use [Git Gateway](https://github.com/netlify/git-gateway) without Netlify by setting up your own Git Gateway server and connecting it with your own instance of [GoTrue](https://www.gotrueapi.org) (the open source microservice that powers Netlify Identity), or with any other identity service that can issue JSON Web Tokens (JWT).
+
+To configure in Netlify CMS, use the same `backend` settings in your `config.yml` file as described in Step 2 of the [Git Gateway with Netlify Identity](#git-gateway-with-netlify-identity) instructions above.
+
+## GitHub Backend
+
+The GitHub backend allows CMS users to log in directly with their GitHub account. Note that the
+user's GitHub account must have push access to your content repository for this to work.
+
+Because Github [requires a
+server](https://github.com/netlify/netlify-cms/issues/663#issuecomment-335023723) for
+authentication, Netlify facilitates basic GitHub authentication.
+
+To enable it:
+
+1. Follow the authentication provider setup steps in the [Netlify
+   docs](https://www.netlify.com/docs/authentication-providers/#using-an-authentication-provider).
+2. Add the following lines to your `config.yml` file:
+
+    ``` yaml
+    backend:
+      name: github
+      repo: owner-name/repo-name # Path to your Github repository
+    ```
+
+
+### External OAuth Clients
+If you would like to facilitate your own OAuth authentication rather than use Netlify's service, you
+can use one of the community-maintained projects below. Feel free to [submit a pull request](https://github.com/netlify/netlify-cms/blob/master/CONTRIBUTING.md) if you'd like to add yours!
+
+
+| Author     | Supported Git hosts       | Languages | Link                                                                |
+|------------|---------------------------|-----------|---------------------------------------------------------------------|
+| @vencax    | GitHub, GitHub Enterprise | Node.js   | [Repo](https://github.com/vencax/netlify-cms-github-oauth-provider) |
+
+Check each project's documentation for instructions on how to configure it.
+
+
+## Bitbucket and GitLab Support
+
+Netlify CMS is meant to be platform agnostic, so we’re always looking to expand the ecosystem and
+find new ways to use it. Check out our active PRs in progress for
+[Bitbucket](https://github.com/netlify/netlify-cms/pull/525) and
+[Gitlab](https://github.com/netlify/netlify-cms/pull/517) backends.
+
+Git Gateway could also be modified to support these Git hosts. If you're interested, you can file an
+issue (or a pull request!) in the [git-gateway repo](https://github.com/netlify/git-gateway).
+
+## Options
+
+Both `git-gateway` and `github` backends allow some additional optional fields for certain use
+cases. A full reference is below. Note that these are properties of the `backend` field, and should
+be nested under that field.
+
+| Field          | Default                                                           | Description                                                                                                                                          |
+|----------------|-------------------------------------------------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------|
+| `repo`         | none                                                              | **Required** for `github` backend; ignored by `git-gateway`. Follows the pattern `[org-or-username]/[repo-name]`.                                    |
+| `accept_roles` | none                                                              | `git-gateway` only. Limits CMS access to your defined array of user roles. Omitting this field gives access to all registered users.                 |
+| `branch`       | `master`                                                          | The branch where published content is stored. All CMS commits and PRs are made to this branch.                                                       |
+| `api_root`     | `https://api.github.com` (ignored for `git-gateway` backend)      | The API endpoint. Only necessary in certain cases, like with GitHub Enterprise.                                                                      |
+| `site_domain`  | `location.hostname` (or `cms.netlify.com` when on `localhost`)    | Sets the `site_id` query param sent to the API endpoint. Non-Netlify auth setups will often need to set this for local development to work properly. |
+| `base_url`     | `https://api.netlify.com`                                         | OAuth client URL for the `github` backend. **Required** when using an external OAuth server with the `github` backend.                               |