add readthedocs config file so dependencies get set, see https://docs…

….readthedocs.io/en/stable/guides/specifying-dependencies.html

readthedocs.yml

@@ -0,0 +1,12 @@
+# config file format version (required)
+version: 2
+
+# build the documentation with Sphinx from the source/ directory
+sphinx:
+  configuration: source/conf.py
+
+# version of Python and requirements required to build the docs
+python:
+  version: 3.8
+  install:
+    - requirements: requirements.txt