add tss -> dcp migration guide (#845)

Summary:
Pull Request resolved: #845

Adds migration guide from TSS -> DCP. Also touches up few things like adding DCP to all.py, etc

Reviewed By: diego-urgell

Differential Revision: D58377638

fbshipit-source-id: 97a01a01fb030c5d33d8ef04d46a191355a9cbb3

Author: JKSenthil

docs/source/index.rst

@@ -49,6 +49,7 @@ Documentation
       meta/getting_started
       meta/migrating
       meta/migrating_example
+      meta/tss_to_dcp
 
 .. toctree::
    :maxdepth: 1