Added option to add canonical link to dektop site.

Websites with separate URLs for desktop and mobile websites need to have a canonical link on mobile site pointing back to the desktop version for SEO.
More Information: https://developers.google.com/webmasters/smartphone-sites/details#separateurls
Option to add canonical link to desktop site.

Updated docs for adding canonical link to desktop site.
Merge pull request #204 from abhinayrathore/master

Option to add canonical link to desktop site.