Document the release process (#2503)

MAINTENANCE.md

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+Creating a release
+==================
+
+The release of the new version of Pyenv is done via GitHub Releases.
+
+Release checklist:
+* Start [drafting a new release on GitHub](https://github.com/pyenv/pyenv/releases) to generate a summary of changes. Save the summary locally.
+  * The summary may need editing. E.g. rephrase entries, delete/merge entries that are too minor or irrelevant to the users (e.g. typo fixes, CI)
+* Push the version number in `libexec/pyenv---version`
+  * Minor version is pushed if there are significant functional changes (not e.g. bugfixes/formula adaptations/supporting niche use cases).
+  * Major version is pushed if there are breaking changes
+* Update `CHANGELOG.md` with the new version number and the edited summary (only the changes section), reformatting it like the rest of the changelog sections
+* Commit the changes locally into `master`
+* Create a new tag with the new version number and push the changes including the tag
+* Create a new release on GitHub based on the tag, using the saved summary