Add README to docs/

Signed-off-by: John Pennycook <john.pennycook@intel.com>

Author: Pennycook

docs/README.md

@@ -0,0 +1,8 @@
+The HTML documentation is built with Sphinx, which can be installed by
+following [these
+instructions](https://www.sphinx-doc.org/en/master/usage/installation.html).
+
+We use the [furo](https://github.com/pradyunsg/furo) theme, which must be
+installed separately.
+
+Other configuration options are documented in source/conf.py.