update readme to add caution around service symbol collision

[mjheilmann]

Because services are combined into a single reflection state object, and because we are divorced from the actual protoc process, there is a risk that we have two services we want to support reflection on that could collide. I'd think if two services were fed to protoc separately the built-in checks could be bypassed and fed to us directly.

In this PR:

• add a callout notifying of this risk in the README
• update State to detect and raise on this data conflict condition
• add tests over the changes to State