Improve release docs ensuring stable official release branch

[twelch]

Improve release instructions to ensure release branch is maintained.

• Is this a bug fix, new functionality, or a breaking change?

Improve release instructions avoid release bugs

• Have read and followed the steps for preparing a pull request.

[smallsaucepan]

Suggested some wording changes @twelch. If you think they're valuable feel free to incorporate.

Also, is there any way we can stop the release branch from being auto-deleted? Seems weirdly forceful of Github.

[twelch]

Thanks for your feedback @smallsaucepan