added extra sinopia config to support packages (facebook#20789)

Summary:
[DOCS] [BUGFIX] [react-native-cli/README.md] - fixed sinopia configuration docs

now that babel uses scoped packages the extra sinpoia config also
requires the rule to cover ** which will satisfy such repositories like
babel/core

The only other issue I had with getting started using this doc was node-gyp being an absolute horror to get running (a dependancy of sinpoia) I don't really know if writing debugging tips/links for macos in these docs is relevant.
Pull Request resolved: facebook#20789

Differential Revision: D9468340

Pulled By: hramos

fbshipit-source-id: c7a41f384655b36e5e46b39d1e7e14313cab9f68

react-native-cli/README.md

@@ -35,6 +35,10 @@ Running it for the first time creates a default config file. Open `~/.config/sin
         allow_access: $all
         allow_publish: $all
 
+      '**':
+        allow_access: $all
+        proxy: npmjs
+
       '*':
         allow_access: $all
         proxy: npmjs