add migration example to docs (#706)

Summary:
Pull Request resolved: #706

Adds a specific migration example to docs, per request of user. Walks through line-by-line how to generally migrate, should help users further.

Reviewed By: anshulverma

Differential Revision: D53869088

fbshipit-source-id: 0d164e930383fa75b11f94686601d68ffd408daa

docs/source/index.rst

@@ -48,6 +48,7 @@ Documentation
 
       meta/getting_started
       meta/migrating
+      meta/migrating_example
 
 .. toctree::
    :maxdepth: 1