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.
Container filesystem initialization and address registration are the canonical use cases for PreStart.

We should support exec. We should make it possible. As proposed in #140 and in various Docker issues, this should execute after the container's filesystem has been constructed but before the entrypoint has been executed.

Also, http handlers won't be useful without some parameters, such as enough info to know which container it is, which pod, labels, IP address, etc.