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.
How do you expect people to make use of this, given the variable amount of time for the lead process in the container to reach a state where it is ready to react to a signal?