Update README and CHANGELOG

CHANGELOG.md

@@ -3,6 +3,9 @@
 * Add `--exclude-migration-tests` option to ignore backward incompatible migration tests.
 * Do not falsely detect dropping `NOT NULL`  constraint.
 * Detect `DROP TABLE` backward incompatible migration.
+* Detect adding a `UNIQUE` constraint as backward incompatible migration.
+* Handle when Django tries to generate the SQL of a migration but raises an exception because
+it cannot find the name of a certain constraint (because it has been dropped already).
 
 Internal change:
 

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.