Add special section for Mac OS X users.
Make installation explicit and discrete, combining suggestions from @thesteve0, @tshepang, @mcdonc.
Merge commit '2ca9bde9d2862655ddee276cd14a375e6a5adec6'
Merge pull request #927 from stevepiercy/master

Add special section for Mac OS X users to Pyramid Install; Make installation explicit and discrete, combining suggestions from @thesteve0, @tshepang, @mcdonc