Expose the projectRoots option

Summary: This was already implemented -- just documenting it in the CLI.
Closes facebook/react-native#3246

Reviewed By: svcscm

Differential Revision: D2560211

Pulled By: martinbigio

fb-gh-sync-id: c4e4d974db28d6251636ff38bfcb0afdf5148de7

packager.js

@@ -41,6 +41,10 @@ var options = parseCommandLine([{
   command: 'root',
   type: 'string',
   description: 'add another root(s) to be used by the packager in this project',
+}, {
+  command: 'projectRoots',
+  type: 'string',
+  description: 'override the root(s) to be used by the packager',
 }, {
   command: 'assetRoots',
   type: 'string',