Add a README for Remote Key Provisioning.

This document goes a little more in depth on the motivating factors and
background mechanisms that occur with RKP, that are not appropriate for
direct inclusion in the HAL docs in the .aidl files.

Fixes: 234159998
Test: Readable
Merged-In: I141fb098c536a5468b1113af64dcf6185ea7ae9f
Change-Id: I141fb098c536a5468b1113af64dcf6185ea7ae9f