Adding docs about how to build a Monogame PCL
Adding docs about how to build a Monogame PCL

rafaelalmeidatk/MonoGame.Extended,cra0zy/MonoGame.Extended,WardBenjamin/MonoGame.Extended,Aurioch/MonoGame.Extended,LithiumToast/MonoGame.Extended,rafaelalmeidatk/MonoGame.Extended,HyperionMT/MonoGame.Extended