Add Sphinx theme dependency in readme

docs/README

 The documents are based on Sphinx, run the following commands to build the documents:
 
-pip install sphinx sphinx-autobuild
+pip install sphinx sphinx-autobuild sphinx-rtd-theme
 pip install recommonmark
 make html
 
-After building, the generated documents are located in _build.
\ No newline at end of file
+After building, the generated documents are located in _build.