[WIP] Add A Readme

[Shelob9]

This is a work in progress PR for discussion.

The readmes in the example plugins will need updating to match. I'll wait on that until this is merged so we can have a consistent pattern.

Things We Should Consider Adding

• Quick documentation of how to make your own plugin with WP CLI - we have a section in the course on this we can link to.
• A list of contributors.
• Links to real plugins that have blocks in them for reference.
• Installation instructions.
• A note to non-developers.

Checklist

• Proofreading
• Update all links to course materials. Everything that should point to a part of the course are currently pointing to learn.wordpress.org, I can get more specific

[pento]

Let's refer to it as "the Block Editor" as much as possible, rather than "Gutenberg".

Also, there are a few different spellings in here, "meta boxes" is the correct way.

[pento]

We're not going to call it Gutenberg Backpack.

I'm leaning towards "Developer's Guide to the Block Editor", to describe what the course is, and future-proofs the name: it's not going to be called Gutenberg when it lands in Core.