Add a CHANGELOG.md & adopt a formatting convention for release notes

[virtualtam]

See http://keepachangelog.com/en/0.3.0/

Proposition:

• adopt a formatting convention for our release notes
• provide a Markdown-formatted CHANGELOG with the full release history

This will help maintainers, users, plugin and template developers to:

• keep track of important changes happening on the Shaarli codebase and documentation
• get this information locally, without need to browse release notes on GitHub

As we're going towards major changes, I think it becomes important to know in a glance which features have been deprecated or removed, and where to find updated configuration/upgrade instructions.

I've already started to refactor pre-v0.5.0 release notes, and wrote a Python script to programmatically generate a Markdown CHANGELOG from GitHub release notes so we have a starting point.

[ArthurHoaro]

OK, this format doesn't have too many constraints. 👍

[virtualtam]

Cool, I'll update the remaining release notes and submit a PR with the resulting change log ;-)