Add link to register renderer

[didiepetrova]

It is not clear the dev must register a renderer to get the content populated, unless he goes through the next articles as well.
A simple line with link to the existing article would be great.

This has already been mentioned as a last step here: http://docs.telerik.com/devtools/xamarin/installation-and-deployment/windows/getting-started-windows

[PavelRPavlov]

with R2 there is no need to manually register renderers