Add release documentation

[JustinyAhin]

Summary

This PR adds a new file Releasing-the-plugin.md with the release steps.

Fixes #131.

Checklist

• PR has either [Focus] or Infrastructure label.
• PR has a [Type] label.
• PR has a milestone or the no milestone label.

[JustinyAhin]

@felixarntz @jeffpaul here is a draft for the release doc. There are still details to add, but I want us to agree on what's included before.

For now, here what the structure looks like:

• Update version number
  • package.json
  • load.php
  • readme.txt
• Update readme.txt
  • npm run readme -- -m "version"
• Create a new GitHub release
• ...

Any other things we should include/change?

[felixarntz]

@JustinyAhin This is a good start, however I think there are a few things missing, and IMO it would be worth clarifying a couple things in more depth, particularly around the branches to use and also to keep other maintainers up to date on the process.

I've added a couple suggestions, let me know your thoughts.

[JustinyAhin]

I've added a couple suggestions, let me know your thoughts.

@felixarntz thanks for looking. All your suggestions make sense. I've committed them to the PR and added a small change.

[felixarntz]

@JustinyAhin Thank you, lgtm! I just added one more comment for your consideration, but it's not critical.