Add logging intro, see #2737
(cherry picked from commit 5fcde199d818e22e6dad4aa70f351a52cefe446f)

Conflicts:

	akka-docs/rst/java/logging.rst
	akka-docs/rst/scala/logging.rst
Merge pull request #1188 from akka/wip-2737-logging-intro-master-patriknw

Add logging intro, see #2737