Add warning regarding feature set, link to features and plugin compat pages

Also sneak in a tiny formatting fix because why not

![warning screenshot](http://dump.dequis.org/WvIG-.png)

>Glowstone is still in heavy development!
>
>A lot of vanilla features are not implemented yet. There are no mobs, no redstone, no physics, but there are enough features for creative servers.
>
>Additionally, some bukkit plugins may not work out of the box.
>
>See the features and plugin compatibility pages for more details.

![navbar screenshot](http://dump.dequis.org/t5x4a.png)