Consider adding changelog to repository

Every time a new wayland-protocols version is released, I find myself searching the commit history to try to figure out what new things have been introduced/changed.

It would be nice to start a changelog in repo so there is a simple spot where all the changes and additions are noted to ensure nothing is missed.

There is definitely a question still regarding how to structure the changelog(s). Does each protocol maintain its own changelog or do we keep a single changelog in repo? If each protocol maintains a changelog, it probably makes sense for the whole repo changelog to mention to mention the protocols with changes or new features.