[TASK] Add a release checklist

Copied without changes (except for the link to the releases list)
from our sister project.

Part of #489.

Author: oliverklee

docs/release-checklist.md

@@ -0,0 +1,15 @@
+# Steps to release a new version
+
+1. In the [composer.json](../composer.json), update the `branch-alias` entry to
+   point to the release _after_ the upcoming release.
+1. In the [CHANGELOG.md](../CHANGELOG.md), create a new section with subheadings
+   for changes _after_ the upcoming release, set the version number for the
+   upcoming release, and remove any empty sections.
+1. Update the target milestone in the Dependabot configuration.
+1. Create a pull request "Prepare release of version x.y.z" with those changes.
+1. Have the pull request reviewed and merged.
+1. Tag the new release.
+1. In the
+   [Releases tab](https://github.com/MyIntervals/PHP-CSS-Parser/releases),
+   create a new release and copy the change log entries to the new release.
+1. Post about the new release on social media.