Create a docs page on adding a blog post to gatsbyjs.org

[m-allanson]

Summary

The Gatsby blog often has guest posts. There are a few specific steps needed to do this, but no documentation on what they are. Add some docs on how to add a post to the site.

Basic example

This should cover at least:

• adding the markdown file with the blog post
• adding meta data as frontmatter
• adding author info
• adding images

Here's an example post that does all of the above

Motivation

Contributors should have clear guidance on how to add a blog post.

[daydream05]

Do guest posts need to get in touch with the Gatsby team first before opening a pull request to add their markdown?

[KyleAMathews]

That'd be a good thing to add to the docs page :-)

Yeah, generally you should file an issue about adding a blog post first if you haven't already discussed things with a core member somewhere.

[KyleAMathews]

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