Sp explanation added

Hi,
I added some explanation of SP given by Jeff in Nupic ML.
This is the first time in my life to play with TeX and I'm still struggling to build TeX environment on Windows7,
so the change might be premature.  Please comment for anything to fix.  or please pull.
At least I verified the added part can be compiled on my laptop.

If it seems okay, I can commit changes more quickly next time. :)