Mention the CI changes

Author: nedbat

CHANGES.rst

@@ -27,6 +27,8 @@ Unreleased
 - When using ``--source`` on a large source tree, v5.x was slower than previous
   versions.  This performance regression is now fixed, closing `issue 1037`_.
 
+- Continuous integration has moved from Travis and AppVeyor to GitHub Actions.
+
 .. _issue 1037: https://github.com/nedbat/coveragepy/issues/1037