Add Trunking configuration

[Fishbowler]

Adds a template that includes and demonstrates Openfire 4.8's trunking capability, to pass traffic from server A -> server C, via server B

[guusdk]

This does not only add a trunking environment, but also a simple and a proxy environment. Should those go in separate PRs? (What do these environments add, functionality-wise? They are adding to the maintenance of everything in this repo.)

The trunking readme does not seem to include the third XMPP server in its ascii-art, and probably can use a description of what functionality can be expected.

[Fishbowler]

Sorry - the Simple and Proxy version come from #51, on which this is based. Simple and Proxy give templates on which to base new more complex environments (e.g. simple + LDAP server + config = LDAP test env)

Sorry again, I'll fix them docs 🤦

[Fishbowler]

Docs still aren't correct, and why are there unrelated plugin in binaries in here??!