Link to mapping .Net PCL Profiles to NuGet Targets

Adds a small new section to the Creating and Publishing a Package page
which provides some links to external resources that help users
determine which NuGet target to use for specific PCL Profiles.

Also fixes a couple of minor typos and deletes a clearly obsolete line
about only supporting Silverlight 3 for Windows Phone
Merge pull request #189 from rvesse/master

Link to mapping .Net PCL Profiles to NuGet Targets