Update README for Control Plane

[drrt]

From @y2bishop2y on August 9, 2017 19:17

Have to explain how to build, run and configure the ControlPlane.

• Configuration
• How to build
• How to deploy
• How to run in a typical configuration
• What the Control Plane does
• Interactions between different components in a SPIFFE deployment
• Description of the plugins
• Registration API Protobuf and Readme.md

Copied from original issue: spiffe/control-plane#45

[drrt]

consolidating into main README