Add note about installing watchman (facebook#1950)

* Add note about installing watchman

* Update CONTRIBUTING.md

CONTRIBUTING.md

@@ -77,6 +77,8 @@ All functionality must be retained (and configuration given to the user) if they
 
 2. Run `npm install` in the root `create-react-app` folder.
 
+3. *(Only for macOS Sierra)*: Until [0.10.0](https://github.com/facebookincubator/create-react-app/milestone/23) is released, you may need to install [Watchman](https://facebook.github.io/watchman/docs/install.html) (e.g. `brew install watchman`).
+
 Once it is done, you can modify any file locally and run `npm start`, `npm test` or `npm run build` just like in a generated project.
 
 If you want to try out the end-to-end flow with the global CLI, you can do this too: