Add a requirements file for readthedocs

JJB docs are also published on readthedocs.org however it currently
fails to build because RTD does not plan to support passing in mulitple
requirements files[1] therefore we need to create a single requirements
file to work with RTD.

[1] https://github.com/rtfd/readthedocs.org/issues/814#issuecomment-125122305

Change-Id: I9574521e33c8f4ad33437c0df5fdcaf8792baae4

docs-requirements.txt

@@ -0,0 +1,2 @@
+-r requirements.txt
+-r test-requirements.txt