Rework "Usage with IDEs" section
Rework "Usage with IDEs" section

Add note about Buildship, and make it clearer that the idea and/or
eclipse plugin(s) need to be applied even when not using their tasks,
and add note about IntelliJ IDEA 2016.1+ and its new "separate module
per source set" behavior.

tbroyer/gradle-apt-plugin,tbroyer/gradle-apt-plugin