Improve documentation on how to rebuild the client on AWS.

[emclaren]

This came up on the Hubs Discussions forum so I'm recording it here for our records - Hubs-Foundation/hubs#2982

Thank you for the positive feedback! If I could throw in my bit on this, a simple document stating how to rebuild the client on AWS would go a long way for me. Thinking ahead, that would be really useful for many people. Right now, this document is not enough - its a vital subject, I get to half way the cant go forward after the prerequisites section: https://hubs.mozilla.com/docs/hubs-cloud-custom-clients.html
It would be nice to have a document stating the steps and the requirements.
Again, thank you for your interest.