README: document the documentation

Author: welshjf

README

@@ -110,3 +110,15 @@ Under bitcoin/tests using test data from Bitcoin Core. To run them:
     python -m unittest discover && python3 -m unittest discover
 
 Please run the tests on both Python2 and Python3 for your pull-reqs!
+
+
+Documentation
+-------------
+
+Sphinx documentation is in the "doc" subdirectory. Run "make help" from there
+to see how to build. You will need the Python "sphinx" package installed.
+
+Currently this is just API documentation generated from the code and
+docstrings. Higher level written docs would be useful, perhaps starting with
+much of this README. Pages are written in reStructuredText and linked from
+index.rst.