Update README with info about changelog and BC breaks

[ghost]

HI. This repository does not contan neither CHANGELOG.md nor UPGRADE.md file. I understand that you use GItHub releases as changelog. However I would add a note about that in README.md.

Moreover, many would appreciate explicit information how BC breaks are denoted. This package still hasn't the first major release so every next 0.x release should be considered unstable. However the release notes do not contain any BC BREAK prefix/section and I'm confused whether it's because there haven't been any yet or you just don't add this prefix because every 0.x release should be treated as unstable.

Proposal:
The changelog is maintained with GitHub releases page https://github.com/thephpleague/oauth2-server-bundle/releases.
Backward compatibility breaks are prefixed with [BC BREAK].

[chalasr]

Agreed. PR Welcome in that direction