Document release process

[jrfnl]

Repost from squizlabs/PHP_CodeSniffer#3861:

In a number of projects I'm involved in, I've started adding "release checklists" in an effort to document the steps for releasing a new version and make that knowledge transferable.

Example:

• https://github.com/PHPCSStandards/PHPCSUtils/blob/develop/.github/release-checklist.md

The release process for this repo is currently not documented and I believe it would be helpful to document it.

Aside from the obvious (tag & release on GH), I'm aware of a few additional things which are needed for this repo, but I currently don't have insight into the exact steps involved/the how of these things.

• Release to PEAR.
• Make the PHAR files available on a web-accessible URL.

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Releasing from PEAR has been dropped in #4.
Other than that, the release process should be documentable.