readme: Expand how to tag commit messages

It's a common mistake that links to issues being resolved are not URLs,
and that Part-of was added manually by contributors. Make it clear what
the expectations are regarding this. Formalize on `Fixes:` for fixing
commit messages, and `Closes:` for closing issues while at it. Format
and alias for generating `Fixes:` tags come from mesa.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2847>

README.md

@@ -49,6 +49,45 @@ commit subjects with a relevant topic, such as `compositor:` or
 `clutter/actor:`, and it's always better to write too much in the commit
 message body than too little.
 
+If a commit fixes an issue and that issue should be closed, add URL to it in
+the bottom of the commit message and prefix with `Closes:`.
+
+Do not add any `Part-of:` line, as that will be handled automatically when
+merging.
+
+### The Fixes tag
+
+If a commit fixes a regression caused by a particular commit, it can be marked
+with the `Fixes:` tag. To produce such a tag, use
+
+```
+git show -s --pretty='format:Fixes: %h (\"%s\")' <COMMIT>
+```
+
+or create an alias
+
+```
+git config --global alias.fixes "show -s --pretty='format:Fixes: %h (\"%s\")'"
+```
+
+and then use
+
+```
+git fixes <COMMIT>
+```
+
+### Example
+
+```
+compositor: Also consider dark matter when calculating paint volume
+
+Ignoring dark matter when calculating the paint volume missed the case where
+compositing happens in complete vacuum.
+
+Fixes: 123abc123ab ("compositor: Calculate paint volume ourselves")
+Closes: https://gitlab.gnome.org/GNOME/mutter/-/issues/1234
+```
+
 ## Default branch
 
 The default development branch is `main`. If you still have a local