Added the how to chat section to readme

I thought I'd add some documentation around how to initiate a chat in the client browser window.  

If this doesn't fit your needs, please advise changes or reject as you see fit.

Thanks,
_m
as a small correction, you can chat to others publicly by clicking on their name too - really depends on whether you want it to seem like it's coming from you to everyone, or directed publicly to someone.