chore: follow release process for existing schema files

Author: james-garner-canonical

juju/client/SCHEMAS.md

@@ -0,0 +1,10 @@
+This file documents the juju release schemas supported by python-libjuju.
+Please see docs/CONTRIBUTING.md for the process for updating this file.
+
+# 3.1
+3.1.0
+3.1.1
+3.1.2 (identical to 3.1.1)
+
+# 3.3
+3.3.0