Add contributing.md information.

Change-Id: I93b4943e116bd268b63a45cd20f8a90fd035ff74
Add timestamp_ms and type Custom to protobuf.

This implements version 0.0.4 of the protobuf-based exchange format.

Change-Id: Ib5b5ea85e00b647877031d2b05ed23352ac0c598
Change type CUSTOM into UNTYPED.

Change-Id: I407fbd2fd9b9482db34962a49761e8a089f279ab
Add ruby support