Add changelog file

CHANGELOG.rst:
Add file to track changes (at least for v51).

CHANGELOG.rst

+#########
+Changelog
+#########
+
+[51] - 2021-02-01
+=================
+
+Added
+-----
+
+- VNC support for `run_archiso`
+- SSH enabled by default in baseline and releng profiles
+- Add cloud-init support to baseline and releng profiles
+- Add simple port forwarding to `run_archiso` to allow testing of SSH
+- Add support for loading cloud-init user data images to `run_archiso`
+- Add version information to images generated with `mkarchiso`
+- Use pacman hooks for things previously done in `customize_airootfs.sh` (e.g. generating locale, uncommenting mirror
+  list)
+- Add network setup for the baseline profile
+- Add scripts for CI to build the baseline and releng profiles automatically
+
+Changed
+-------
+
+- Change upstream URL in vendored profiles to archlinux.org
+- Reduce the amount of sed calls in mkarchiso
+- Fix typos in `mkarchiso`
+- mkinitcpio-archiso: Remove resolv.conf before copy to circumvent its use
+- Remove `customize_airootfs.sh` from the vendored profiles
+- Support overriding more variables in `profiledef.sh` and refactor their use in `mkarchiso`
+- Cleanup unused code in `run_archiso`