Describe safe_by_default behaveiour explicitly (#229)

README.md

@@ -730,7 +730,7 @@ Certain methods like `execute` and `change_table` cannot be inspected and are pr
 
 ## Safe by Default
 
-Make operations safe by default.
+Make operations safe by default. For these select migrations, this allows you to write the code under the "Bad" section, but the migration will be performed as if you had written the "Good" version.
 
 - adding and removing an index
 - adding a foreign key