Keep a change log

It would be neat if a change log was kept in the repository or the documentation so that I, as a user or distribution package maintainer, can get an overview of new capabilities and changed behaviour in new versions without having to read through all commits.

The problem with reading through commit logs is that they are noisy and written from the perspective of how the code changes. A change log could focus on what changes for a user. E.g, the entry for version 2.7.0 could look like this:

* Support customizing or disabling the notification icon.
* Support cusotmizing the notification timeout.
* Have auto-ntfy ignore `htop` by default.
* Make auto-ntfy send notifications in the background so that control returns to the shell faster.