PYTHON-706 Mention that backports.pbkdf2 provides the best performance.

(cherry picked from commit a7db21a)

doc/examples/authentication.rst

@@ -22,6 +22,11 @@ passed to the :meth:`~pymongo.database.Database.authenticate` method::
   >>> uri = "mongodb://user:password@example.com/the_database?authMechanism=SCRAM-SHA-1"
   >>> client = MongoClient(uri)
 
+For best performance install `backports.pbkdf2`_, especially on Python older
+than 2.7.8, or on Python 3 before Python 3.4.
+
+.. _backports.pbkdf2: https://pypi.python.org/pypi/backports.pbkdf2/
+
 MONGODB-CR
 ----------