=doc #3500 Add section about Dependency Injection
(cherry picked from commit 2087dfc1a0ba04e4889216d938d25794b32f2d2d)

Conflicts:
	akka-docs/rst/scala/howto.rst
Merge pull request #1684 from akka/wip-3500-doc-di-2.2-patriknw

=doc #3500 Add section about Dependency Injection (backport)