Add .watchmanconfig

Summary:
Watchman on facebook corporate machines is configured to only start on directories with a `.watchmanconfig` file (`enforce_root_files` is true, see https://facebook.github.io/watchman/docs/config.html).

Adding this allow `jest` to run in watch mode on Facebook machines.
Closes #2408

Reviewed By: jstejada

Differential Revision: D7550102

Pulled By: kassens

fbshipit-source-id: 17572918c0b5fd3e2a5f8877233952d8a7446531