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.
My thought is something like:

1) Main entry point blocks for some signal.
2) PostStart does some registration, and sends the signal
3) Main entry point continues.