More info in README

[lacarvalho91]

I was recently asked some questions about how the KMS works that weren't answered by the README, specifically whether or not the schedules are held in memory.

There was also a problem reported recently which turned out to be because the user was sending null key values to the input topic, we should why that can't happen.

And some numbers too, in prod we see 9mil schedules using 3.2gb heap.

Add information on the following:

• Architecture of the KMS with Sequence Diagram
• How to run in Production