Set up separate cli tool

Summary:
Discussed that we want this to be both a library and a CLI
tool. Also added a README stub.

Reviewed By: nikoant

Differential Revision: D19969788

fbshipit-source-id: 2c54935ce5953d2053c7bdf601debe0e5e911647

pkg/src/index.ts

@@ -7,6 +7,4 @@
  * @format
  */
 
-(async () => {
-  console.log('Hello, flipper-pkg.');
-})();
+export const PKG = 'flipper-pkg';