Added help section for chat

I added a chat section in help page. Please let me know what you want to put inside.
I think you can be more verbose here, and add "XMPP is a standrad protocol for chating, that means the diaspora* interface is not the only way to connect to the chat. You can connect with any XMPP client you want, desktop or online, using our diaspora* credential, and add friends from other XMPP server, like jabber or even gmail."

But we probably need @goob to review my english here.