=doc Link to CONTRIBUTING.md and remove obsolete dev guidelines
Merge pull request #16017 from akka/wip-doc-contributing-patriknw

=doc Link to CONTRIBUTING.md and remove obsolete dev guidelines
=pro Grab binary version for MIMA combatibility testing from a setting.

Additionally, also use Akka 2.3.2 for checking Scala 2.11 binary
compatibility as there was no Akka 2.3.0.
Merge pull request #16137 from jsuereth/wip/mima-fixins

Fix the definition of akkaPreviousArtifact so it uses the build's scalaVersion correctly
=pro enable cross builds

* changed zeromq-scala-binding library
* added dependency helpers from master by Johannes.
* modified build-release task to include distributions from cross builds
* removed tar-zip step from release script, because tgz package is not used from the website
* release script now releases for all crossScalaVersions
* dry run is the default now