It is possible to hook up in-tree documentation to Sphinx. Convert the
one-off README.md to ReStructuredText and add it to the Sphinx docs.
I added a moz.build file under testing/ because I don't think it is
appropriate for the Sphinx directive to live in the root moz.build file.
MozReview-Commit-ID: 90tCb7mA63C
--HG--
rename : testing/taskcluster/README.md => testing/taskcluster/docs/index.rst
extra : rebase_source : 9312445aa17a1a2b03195a4f18442f8a8a0d8fe9
