add requirements for readthedocs (#341)

* add requirements for readthedocs

docs/conf.py

@@ -13,6 +13,7 @@
 from recommonmark.transform import AutoStructify
 from recommonmark.parser import CommonMarkParser
 import os
+import paddlehub as hub
 import sys
 sys.path.insert(0, os.path.abspath('../paddlehub'))
 

docs/requirements.txt

+sphinx==2.1.2
+sphinx-argparse==0.2.5
+sphinx-markdown-tables==0.0.12
+sphinx-rtd-theme==0.4.3
+recommonmark==0.6.0
+paddlehub