Fix readthedocs.io build process (#1004)

* fix readthedocs.io build process

docs/requirements.txt

@@ -0,0 +1 @@
+mkdocs==1.1

readthedocs.yml

@@ -0,0 +1,22 @@
+# 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
+mkdocs:
+  configuration: mkdocs.yml
+  fail_on_warning: false
+
+# Optionally build your docs in additional formats such as PDF
+formats:
+  - pdf
+  - epub
+
+# Optionally set the version of Python and requirements required to build your docs
+python:
+  version: 3.7
+  install:
+    - requirements: docs/requirements.txt