Add some documentation for the container environment.

@smarterclayton @bgrant0607 @thockin @erictune 

We've talked about some of this in the past.  Mostly this describes what is already implemented, but there are some proposals in there as well, please take a look.
By "open whether we support the environment variables or not" do you mean, whether we support adding them automatically to every pod, instead of requiring the pod author to indicate they want them exposed?  Or something else?