Add RELEASE-PROCESS.md

[ac000]

The document describes the release process for NGINX Unit.

[ac000]

Add a section to check for the new release on GH.

$ git range-diff 1637ab23...10100df1
1:  1637ab23 ! 1:  10100df1 Add RELEASE-PROCESS.md
    @@ RELEASE-PROCESS.md (new)
     +    $ git push <upstream> 1.33.0
     +
     +
    ++# A new 'Release'
    ++
    ++After a while the new release should show up at
    ++<https://github.com/nginx/unit/releases>
    ++
    ++
     +# Tarball
     +
     +We need to publish an archive of the source and a checksum.

[avahahn]

Note:

We also need a section to update these example dockerfiles in the docs repo.