Document how to build the documentation

Signed-off-by: Niels Thykier <niels@thykier.net>

doc/contributing-to-britney.rst

@@ -33,8 +33,13 @@ The `runtests` command in `britney2-tests` supports running only a
 subset of the tests.  Please see its `--help` output for more
 information.
 
-Finally, there is also some heavier tests based on some snapshots of
+There are also some heavier tests based on some snapshots of
 live data from Debian.  The data files for these are available in the
 `live-data` submodule of the `britney2-tests` repo.  They consume
 quite a bit of disk space and britney will need at least 1.3GB of RAM
 to process them.
+
+
+Documentation is handled by sphinx and can be built bia::
+
+    make html