updated contributing guide sinopia section to deal with use auth issue

react-native-cli/CONTRIBUTING.md

@@ -14,6 +14,10 @@ Then, open `~/.config/sinopia/config.yaml` and configure it like this (note the
 
     storage: ./storage
 
+    auth:
+      htpasswd:
+        file: ./htpasswd
+
     uplinks:
       npmjs:
         url: https://registry.npmjs.org/
@@ -45,6 +49,7 @@ Now you can run sinopia by simply doing:
 Now we need to publish the two React Native packages to our local registry. To do this, we configure npm to use the new registry, unpublish any existing packages and then publish the new ones:
 
     react-native$ npm set registry http://localhost:4873/
+    react-native$ npm adduser --registry http://localhost:4873/
     # Check that it worked:
     react-native$ npm config list
     react-native$ npm unpublish --force