Add readthedocs configuration

.readthedocs.yml

@@ -0,0 +1,10 @@
+version: 2
+
+sphinx:
+  builder: htmldir
+  configuration: docs/conf.py
+
+python:
+  version: 3.7
+  install:
+    - requirements: docs/requirements.txt

docs/requirements.txt

@@ -0,0 +1,2 @@
+sphinx == 3.2.1
+sphinx-rtd-theme==0.5.0