Add documentation about IDE specific configuration

[Bukama]

In #229 / #262 we provided additional IntelliJ specific configuration. Those needs to be documented.

[nipafx]

When I blabbered about documentation, I meant we could add the commit message configuration to Gradle-for-IntelliJ as aepfli described and that we need to document that. I now realize that there's no point in configuring that, though, because the commit messages written in IntelliJ will never make it into the repo - squash&merge, right?