Bug 1425708 - Link to Marionette README. r=me

DONTBUILD

MozReview-Commit-ID: feWWjE334r

testing/marionette/doc/index.rst

@@ -24,17 +24,18 @@ For users
 .. toctree::
    :maxdepth: 1
 
+   `Marionette README`_
    Intro.md
    Protocol.md
 
-
 See also:
 
-* Documentation for `Marionette Python client`_., which is used
+* Documentation for `Marionette Python client`_, which is used
   in-tree to write many kinds of Marionette-based tests.
 * Documentation for `Firefox Puppeteer`_, which is used to in-tree
   to write Firefox UI tests.
 
+.. _Marionette README: https://searchfox.org/mozilla-central/source/testing/marionette/README.md
 .. _Marionette Python client: http://marionette-client.readthedocs.io
 .. _Firefox Puppeteer: http://firefox-puppeteer.readthedocs.io