Write plugin API documentation

[nsg]

At the moment you need to read the server source, or look at already existing plugins to figure out how to write and install a plugin, this is no good :) We need to write some docs.

[petterarvidsson]

I'd say http://doc.konstructs.org/server-api/index.html takes a step closer to solving this issue. What more should we do?

[nsg]

Guides explaining the basic concepts and how to do things.

[nsg]

Added this to release 8 because it feel like the API has started to stabilize and it's time to write a few guides around that.