PYTHON-884 - Add 3.0 migration guide.

Author: behackett

doc/index.rst

@@ -22,6 +22,9 @@ everything you need to know to use **PyMongo**.
 :doc:`faq`
  Some questions that come up often.
 
+:doc:`migrate-to-pymongo3`
+ A PyMongo 2.x to 3.x migration guide.
+
 :doc:`python3`
  Frequently asked questions about python 3 support.
 
@@ -91,4 +94,5 @@ Indices and tables
  contributors
  changelog
  python3
+ migrate-to-pymongo3
  developer/index