Clarify version scheme

Author: hynek

CHANGELOG.rst

@@ -2,7 +2,10 @@ Changelog
 =========
 
 Versions follow `CalVer <https://calver.org>`_ with a strict backwards compatibility policy.
-The third digit is only for regressions.
+
+The **first digit** of the version is the year.
+The **second digit** is incremented with each release, starting at 1 for each year.
+The **third digit** is when we need to start branches for older releases (only for emergencies).
 
 Put simply, you shouldn't ever be afraid to upgrade ``attrs`` if you're only using its public APIs.
 Whenever there is a need to break compatibility, it is announced here in the changelog, and raises a ``DeprecationWarning`` for a year (if possible) before it's finally really broken.