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.
IMO, PostStart isn't as useful as PreStart, but could be used for self-registration with a discovery service. 

In order to initialize state inside the container, it would need to hand-shake with the application (e.g., via filesystem lock or SIGHUP), in which case the application might as well do the initialization.