CONTRIBUTING: Document the Fixes header.

Document the use of the Fixes header to refer to a commit that
introduced a bug being fixed.

Signed-off-by: Russell Bryant <russell@ovn.org>
Acked-by: Joe Stringer <joe@ovn.org>

CONTRIBUTING.md

@@ -231,6 +231,16 @@ Examples of common tags follow.
         in old change log entries.  (They are obsolete because they do
         not tell the reader what bug tracker is referred to.)
 
+    Fixes: 63bc9fb1c69f (“packets: Reorder CS_* flags to remove gap.”)
+
+        If you would like to record which commit introduced a bug being fixed,
+        you may do that with a “Fixes” header.  This assists in determining
+        which OVS releases have the bug, so the patch can be applied to all
+        affected versions.  The easiest way to generate the header in the
+        proper format is with this git command:
+
+        git log -1 --pretty=format:"Fixes: %h (\"%s\")" --abbrev=12 COMMIT_REF
+
 Developer's Certificate of Origin
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