versioning: create branch 1.0.x
The upcoming release will be called 1.0 rather than 0.11.0. The main motivation for this is that I consider hg-git “ready”. In order for semantic versioning to make sense, something has to eventually be good enough. With the changes to user experience, I personally consider this good enough to actually use. And besides, if this isn't 1.0, when should it be? When development ceases? I've updated `CONTRIBUTING.rst` to specify an explicit compatibility policy.