Document plugin.yml & plugin.groovy

[erichelgeson]

Plugins exclude application.yml/groovy but include plugin.yml/groovy - currently undocumented.

[graemerocher]

not sure if this should be documented. The idea was for plugins to be able to define default values, but in reality most plugins don't use this feature and it simply slows startup time / consumes memory.

[virtualdogbert]

I think it should be documented, but with the warning that it can slow startup/consumes memory.

Although it would be nice to have the option, without a slowdown and consuming more memory... because it's good to have good defaults. that can be overridden, although that could be done in other ways...