Introduce CHANGES.txt

In order to easier create release notes, we should switch to a CHANGES.txt file.

The problem with commits is, that they are written, when the developer is
kneedeep in code, which results in more likely technology focused commits
instead of feature/functionality focused commit messages. This file should
be used to explain, what really happened and what has been fixed from a
user point of view. This file has the huge advantage if being edited after the commit/PR has been finished in order to ensure good and useful changelog messages.

I filled up this file with the releases of 0.90 until now and all the 1.0beta releases