Fix #9 - add information message when altering columns - may be backward compatible

django_migration_linter/sql_analyser.py

@@ -53,7 +53,7 @@ migration_tests = (
     }, {
         'code': 'ALTER_COLUMN',
         'fn': lambda sql, **kw: re.search('ALTER TABLE .* MODIFY', sql),
-        'err_msg': 'ALTERING columns'
+        'err_msg': 'ALTERING columns (Could be backward compatible. You may ignore this migration.)'
     }, {
         'code': '',
         'fn': has_default,