Documenting release branch creation

Whenever we create a new release, we use [the scripts in tools repository](https://github.com/citusdata/tools/tree/develop/automated_packaging) to create changelogs, create release branches, update versions in necessary places etc.

These scripts are not maintained, and have various issues, to the point that making all the changes takes less time than trying to review and fix the commits that those scripts create and push to GitHub.

That's why we need to document and list the required steps to make sure everything goes as planned.

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A preliminary list of things I do when creating the release-9.1 branch is as follows:

- Update `CHANGELOG` in master
- Go over all the descriptions of PRs that are merged since the previous release.
- Create a new `release-9.1` branch from master
- Update versions in both branches (master: `9.1devel` -> `9.2devel`, release: `9.1devel` -> `9.1.0`)
- Update `configure.in` and run `autoconf -f`
- Create new empty migration scripts on master for 9.2 on master
- Update `multi_extension` test outputs, add new upgrade steps when necessary