Document using waffle in contributing.md

[jaredhirsch]

@nchapman we might as well document the use of waffle point estimates at the same time:

• values based on complexity/difficulty, not time
• we use point values of 1,2,3,5, might be good to include examples of each

I'm not yet sure how to estimate epics--maybe we punt on that for now, unless you have ideas.