[gatsby-plugin-sitemap] - update docs

[busticated]

• correct default query description (all pages are included, not just makdown)

• clarify how to override defaultOptions via gatsby-config.js

• add warning: sitemap is not generated when running gatsby develop

[gatsbybot]

Deploy preview ready!

Built with commit f30201a

https://deploy-preview-3234--gatsbygram.netlify.com

[KyleAMathews]

Thanks!

[jlengstorf]

Hiya @busticated! 👋

This is definitely late, but on behalf of the entire Gatsby community, I wanted to say thank you for being here.

Gatsby is built by awesome people like you. Let us say “thanks” in two ways:

1. We’d like to send you some Gatsby swag. As a token of our appreciation, you can go to the Gatsby Swag Store and log in with your GitHub account to get a coupon code good for one free piece of swag. (We’ve got t-shirts and hats, plus some socks that are really razzing our berries right now.)
2. If you’re not already part of it, we just invited you to join the Gatsby organization on GitHub. This will add you to our team of maintainers. You’ll receive an email shortly asking you to confirm. By joining the team, you’ll be able to label issues, review pull requests, and merge approved pull requests.

If you have questions, please don’t hesitate to reach out to us: tweet at @gatsbyjs and we’ll come a-runnin’.

Thanks again! 💪💜

[busticated]

thanks @jlengstorf this is super-cool! ❤️ 👏 👍