This adds a 'watchman.json' file to /tools/tryselect and some documentation on
how to use it. Tl;dr, install watchman and then:
$ cd path/to/gecko
$ watchman -j < tools/tryselect/watchman.json
Differential Revision: https://phabricator.services.mozilla.com/D28771
--HG--
extra : moz-landing-system : lando
Andrew Halberstadt