Add a (very) brief intro

[marksteward]

We used to say this on the homepage:

Welcome to DC4420, aka Defcon London.

.reality [2015 edition]

Meetings are normally the last Tuesday of the month, except December...

New attendees welcome, just come along!
(There are no entry fees, and no tickets to book.)`

Most of it's redundant, but we should probably have at least a link to the About/Where page, as menus suck for some people.