Adding Recipes to README

I originally wrote these for documentation of the Makefile in power_manager, but
it was suggested they would be better served here. I have reformatted them a bit
and make them a bit less power_manager specific.

BUG=chromium-os:30057
TEST=None, this is a documentation only change and thus should have no functional
     effect.

Change-Id: I1e0ee19450e087023b73507ae33725ebba3609ba