geohash/release.txt

Mint a release:

https://opam.ocaml.org/doc/Packaging.html#Publishing

- proof-read Change.txt
- $ cat Change.txt >> Changelog.txt && git commit
- create the git tag with the version as in dune-project:
  
  $ git tag -F Change.txt 0.1
- create, sign and publish the tarball and evtl. binaries (e.g. to git repo hoster)
- $ opam publish <tarball> .
- 🎉
- edit version number in dune-project for next release, prepare Change.txt for the
  next release