Add documentation for implementing a new plugin

[mvaude]

I am currently at the infrakit BoF in Berlin, I am playing around with it for the first time.
I want to create a flavor plugin for a specific stack. I cannot find anywhere documentation for when you are writing a plugin.
I am checking the source code from other flavors trying to miss nothing for my implementation.

It could be great to document a plugin implementation documentation for (future) contributors (with the requirements for a group or instance or flavor plugin).

[linsun]

I second this req :)

[hekaldama]

Maybe even adding a lib that people could include to get going. Similar to https://github.com/docker/go-plugins-helpers

[wfarner]

@hekaldama we have similar libraries for each plugin type, adapting the Go interfaces to HTTP servers. Check it out! https://github.com/docker/infrakit/tree/master/spi/http

[hekaldama]

Nice!