Document the release process of this package

[fingolfin]

We should document the release process of this package. Here is how I think it works (based on my bad memory); I'll update this during the next release and then we can add it to a file in the repository, e.g. etc/README.md or README.maintainer.

1. Switch to the master branch and cd into the root directory of the git repository.

2. Update the version of the Julia and GAP packages by invoking the script etc/update_version.sh with the new version as argument. Example: etc/update_version.sh 0.3.0 (by the way, this is shell script which calls perl right now; it would make sense to rewrite it as a Julia script to avoid the need for perl).

3. git push

4. Ideally wait for Travis CI to complete the full CI tests for the commit you just pushed

5. Comment on the commit on GitHub with the message @JuliaRegistrator register (see here for an example).

The latter works because we have set up a GitHub Action for Julia TagBot to tag and make the release automatically.

[JuliaRegistrator]

Error while trying to register: "Tag with name 0.2.6 already exists and points to a different commit"

[fingolfin]

Ooops, didn't mean to actually trigger the JuliaRegistration bot...

[mohamed-barakat]

This is funny :)