Update readme with new addin repository.

Add addin repository link for Xamarin Studio 4.1

Change Mono version tested on OS X to 2.10.12. Some menus
for Xamarin Studio do not appear when using Mono 2.10.11
with newer versions of Xamarin Studio. Version 4.0.1 on
OSX is OK but 4.0.9 and 4.1.6 had missing menus unless
Mono 2.10.12 was used.