Auto generate Changelog

[cburgdorf]

What is wrong

Manually writing a changelog is error prone and tedious.

How can it be fixed

Even if this isn't a Python project, I believe towncrier currently provides the best system to generate release notes.

If we want a pure rust solution then something like https://github.com/clog-tool/clog-cli can be an alternative (disclaimer: I'm the biased author) but I'm strongly in favor to use towncrier manly because:

• it decouples changelog entries from commits
• it is used in all other snakecharmer projects

[cburgdorf]

In case we want to go with towncrier, one thing that just occured to me is that it defaults to RST output while we need markdown. One can work around that though: twisted/towncrier#128 (comment)

[cburgdorf]

Actually, if we go with towncrier I think I will tackle this before the first release and just write down all the notes by hand that our PRs should have had included.

[cburgdorf]

This landed as part of #157