Document setup process better

* Do I have to run this on the same machine as the matrix server?
* Does this require an xmpp server?
* Do I have to run this on the same machine as the xmpp server?

I just want to talk to xmpp contacts through my existing xmpp account which is on a free xmpp server I do not own. Is this software the best way to do that at the moment?

```
backendOpts: <=== no sure what to put here
# endpoint to reach the component on. The default port is 5347
service: "xmpp://localhost:5347"
# domin assigned to the component.
domain: "matrix.localhost"
# password needed by the component.
password: "jam"

portals:
# List of regexes to match a alias that can be turned into a bridge.
aliases:
# This matches _purple_ followed by anything
"/^_xmpp_(.+)$/":
# Use the prpl-jabber protocol.
protocol: "prpl-jabber" <===== does this need to be changed for xmpp.js?
properties:
# Set room to the first regex match
room: "regex:1"
# Set the server to be conf.localhost
server: "conf.localhost"
```

Start by reviewing the setup configuration shown in the issue, especially backendOpts and the portals settings for service, domain, protocol, room, and server. Verify the machine, XMPP-server, and free-account requirements, then document the supported setup and clarify what each configuration value should contain.

Written by the indexing model from the issue text.