Added documentation for IDEA IDE integration

first shot for IDE integration on contribution page ...

pull request is here. Of course I struggled a little bit. Finally awestruct/sass (or me?) was not able to compile styles.scss so locally the website looks broken.

I´m not sure, where to put the images, you may find a better place for them.

I also added some arrows (->) to indicate menu navigation. I guess it would be nice to use some fancy icons from bootstrap instead