Use releases as an alternative to CHANGELOG.md

[kaittodesk]

Thinking about ditching CHANGELOG.md in favour for documenting changes in releases. At the moment both contain exactly the same information, and there isn't a real value to maintaining both.

One downside would be that plugin's changelog will be tied to GitHub. If we would ever decide to leave GitHub we might not be able to take our releases information with us.

PS. This does not exempt developer from updating readme.txt, as it will be used to populate release information in Wordpress.org.