Added versionchanged docs to add_user

Author: rozza

pymongo/database.py

@@ -549,6 +549,9 @@ def add_user(self, name, password, read_only=False):
  - `password`: the password of the user to create
  - `read_only` (optional): if ``True`` it will make user read only
 
+ .. versionchanged:: 2.1.1+
+ Added support for read only users
+
  .. versionadded:: 1.4
  """
  pwd = helpers._password_digest(name, password)