add doc regarding CGI environment

most common vars in CGI environment explained
Okay. So, `X-Zerocloud-Id` is the id for the job/request. I don't really understand the [comments](https://github.com/zerovm/zerocloud/blob/c6310c74465907bea49773b70dfba4aec2741984/zerocloud/proxyquery.py#L570-572) about this ID. I'm looking at the `thread_pool`, which seems to be the primary consumer of this ID, but it's really clear how all of that works. Anyway, that's another topic. I'm still haven't yet dug into the thread pooling part of ZeroCloud. =)