docs(contributing): add instructions to install grunt-cli/bower globally on unix systems

Closes angular#5814
Closes angular#5811

Author: darkoverlordofdata

docs/content/misc/contribute.ngdoc

@@ -87,6 +87,16 @@ grunt package
 Administrator). This is because `grunt package` creates some symbolic links.
 </div>
 
+<div class="alert alert-warning">
+**Note:** If you're using Linux, and npm install fails with the message 
+'Please try running this command again as root/Administrator.', you may need to globally install grunt and bower:
+<ul>
+<li>sudo npm install -g grunt-cli</li>
+<li>sudo npm install -g bower</li>
+</ul>
+
+</div>
+
 The build output can be located under the `build` directory. It consists of the following files and
 directories: