docs: Add optional sphinx_rtd_theme; add docs build instructions.

The sphinx_rtd_theme is used by ReadTheDocs to render a pretty looking
documentation.  If you have this theme installed locally then your
locally-compiled docs will look exactly like the published
documentation.   Otherwise it falls back to the default theme.

1 files changed, 15 insertions, 0 deletions

diff --git a/README.md b/README.md
index 17e369a3f..56c0ebafb 100644
--- a/README.md
+++ b/README.md
@@ -98,3 +98,18 @@ AUR.  If the above does not work it may be because you don't have the
 correct permissions.  Try then:
 
     $ sudo dfu-util -a 0 -d 0483:df11 -D build-PYBV10/firmware.dfu
+
+Building the documentation locally
+----------------------------------
+
+Install Sphinx, and optionally (for the RTD-styling), sphinx_rtd_theme,
+preferably in a virtualenv:
+
+     pip install sphinx
+     pip install sphinx_rtd_theme
+
+In `micropython/docs`, build the docs:
+
+    make html
+
+You'll find the index page at `micropython/docs/build/html/index.html`.