add yml file

-from here: https://docs.readthedocs.io/en/stable/config-file/v2.html
- because index in url was empty, see here: 
readthedocs/readthedocs.org#3634

Author: urswilke

docs/.readthedocs.yml

@@ -0,0 +1,25 @@
+# .readthedocs.yml
+# Read the Docs configuration file
+# See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
+
+# Required
+version: 2
+
+# Build documentation in the docs/ directory with Sphinx
+sphinx:
+  configuration: docs/conf.py
+
+# Build documentation with MkDocs
+#mkdocs:
+#  configuration: mkdocs.yml
+
+# Optionally build your docs in additional formats such as PDF
+# formats:
+#  - pdf
+
+# Optionally set the version of Python and requirements required to build your docs
+python:
+  version: 3.8
+  setup_py_install: true
+#  install:
+#    - requirements: docs/requirements.txt