Add documentation about how to add new models

[lgeiger]

[AdamHillier]

The method we are currently using to host weights as GitHub release assets is not bad, but we do need to be a bit careful with it.

Each time we trigger a new release to host the weights files we end up triggering our publish action, which tries to publish a new version to PyPI. This fails only because PyPI rejects a package with the same version number in setup.py.

See here for an example.

[lgeiger]

Thanks for catching it.

Hmm this should prevent running the publish action on tags that don't start with v https://github.com/larq/zoo/blob/31d5b47e89167866349ea919534afa26149ff2a2/.github/workflows/publish.yml#L7

Looks like #131 introduced this regression.