Tell me whether master is the latest published version #2601

I think that the objective of https://github.com/sindresorhus/refined-github/issues/288 was to avoid using the wrong version of a readme.

We should have display cue that the user is looking at something that hasn't been released. Such cue would also be useful for contributors that it may be time to tag a new release.

GitHub already includes this information for branches:

<img width="400" alt="" src="https://user-images.githubusercontent.com/1402241/70344101-758e4400-188b-11ea-9ab6-b97ea75d4905.png">

We probably cannot display it the same way because it would cause the content to jump down. Ideally we can merge this info with the current tag button in the branch-buttons feature.

Current branch is bleeding edge

<img width="112" alt="" src="https://user-images.githubusercontent.com/1402241/70344372-2ac0fc00-188c-11ea-9697-4139e9a13dd1.png">

Current branch is latest

<img width="196" alt="" src="https://user-images.githubusercontent.com/1402241/70344480-6c51a700-188c-11ea-8ea2-bbdb7c993e33.png">