Use Town Crier to manage changelog

[kdeldycke]

It seems there is a way to produce Markdown changelogs with towncrier. See: twisted/towncrier#128

This might be helpful to manage version fragments, and have them in sync between:

• Sphinx docs
• GitHub releases
• Copies in Readme showing delta-changes

[kdeldycke]

Producing Markdown with TownCrier has been documented at: https://towncrier.readthedocs.io/en/latest/markdown.html

Refs twisted/towncrier#128 and twisted/towncrier#436

[kdeldycke]

Also look at: https://github.com/callowayproject/generate-changelog