Add an example to install a gruntplugin via npm

Fixes gruntjs/gruntjs.com#139
Add a link to the plugins page on the installing section at the Getting Started page

Fixes gruntjs/gruntjs.com#140
Fix plugins link
Merge pull request #96 from leobalter/gruntplugin-install-example

Add an example to install a gruntplugin via npm