docs(readme): add installation of pub as a pre-requisite

Closes angular#113

Author: hswolff

README.md

@@ -6,6 +6,8 @@
 
 1. `npm install`
 2. `npm install -g gulp karma`
+3. [Install the Dart SDK](https://www.dartlang.org/tools/sdk/)
+4. [Add the Dart SDK's `bin` directory to your system path](https://www.dartlang.org/tools/pub/installing.html)
 3. `gulp build`
 4. `pub get`