added readthedocs yaml

Former-commit-id: 39449cd7

.readthedocs.yml

+version: 2
+
+python:
+  version: 3.7
+  install:
+    - requirements: doc/requirements.txt
+
+sphinx:
+  builder: html
+  configuration: doc/conf.py
+
+submodules:
+  exclude: all