doc: Add paragraph about mkdocs

doc/index.md

@@ -13,3 +13,13 @@ you have installed. If you cloned the repository manually, you can find the
 right documentation in the directory `doc/`. If you're viewing this online at
 <https://restic.readthedocs.org>, there is a small menu at the bottom left of
 this page, where you can select the version.
+
+The restic documentation is built with [MkDocs](http://www.mkdocs.org). After
+installing it, you can edit and view the documentation locally by running:
+
+    $ mkdocs serve
+    INFO    -  Building documentation...
+    INFO    -  Cleaning site directory
+    [I 160221 12:33:57 server:271] Serving on http://127.0.0.1:8000
+
+Afterwards visit the URL with a browser.