Document the GCS protocol better about opengcs

[jterry75]

What are you after here? The message format documentation or the workflow documentation? Both?

[beweedon]

I included this just because it would be a good idea to go through every aspect of the protocol (what each message means, which exact circumstances each message field should be used in and how it should be used, etc.), and see if we could improve it at all to be more straightforward for people without internal knowledge of how the HCS works. It's fine if we decide nothing needs to be added, but we should still look into it to make sure.