Added how to commit and contribute docs to pig wiki

[Alan Gates]

I've added http://wiki.apache.org/pig/HowToContribute and 
http://wiki.apache.org/pig/HowToCommit to the pig wiki.  I lifted these 
straight from the hadoop site, with some editing to reflect the 
differences between the projects.  I'm not saying we have to follow 
hadoop's processes, but they seem like a good starting point for 
discussion.  So let the discussion begin.

Alan.