Document default cache:key

From http://docs.gitlab.com/ce/ci/yaml/README.html#cache-key, I can't tell what the default caching scheme is. From observation, it looks like it creates a cache key of default.

As an aside, while this feels like it might be safe, it means merge requests get slow first builds, which is probably a bad developer experience. Of course having crazy build results would be a bad DX too.