Enable Yarn check files (facebook#3769)

* This is a good default.
Adds approx 4 seconds to install time, but can save some headaches.

* Add no lockfile for add, too

Author: Timer

.yarnrc

@@ -1 +1,3 @@
 --install.no-lockfile true
+--install.check-files true
+--add.no-lockfile true