watchman: EASY: add .projectid file
watchman: EASY: add .projectid file

Summary:
this makes it possible for getdeps to figure out which
project should be built by default.

Reviewed By: chadaustin

Differential Revision: D20740409

fbshipit-source-id: 3838bb90f61cf5358ef0aeaf9cf479ef0d5f1824

facebook/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman