Update README and CHANGELOG

README.rst

@@ -81,6 +81,7 @@ The linter analyses your migrations and checks the SQL for:
 - Renaming columns
 - Renaming tables
 - Altering columns (which can be backward compatible and potentially ignored)
+- Adding a unique constraint
 
 Those are the most important and frequent backward incompatible migrations.
 We are happy to add more if you can specify them to us.