Add comment to README.md

The user needs to have "mongodb" Node.js module installed.

Author: Kirill Zdornyy

README.md

@@ -5,4 +5,6 @@ List, create, read, update and delete documents using AngularJS $resource on the
 
 It is assumed that you have a MongoDB server that does not require authentication running locally on your machine on port 27017. This can always be changed in app.js if you want to use something like MongoLab.
 
+Run "npm install" to install "mongodb" Node.js module dependency.
+
 This code is for instructional purposes only and should not be used in production due to major security inconsiderations.