Unexpected behavior when using Scala string interpolation in g8 templates

This is currently not (simply) supported because `$` signs are interpreted by the templating engine which is configured to use `$` to enclose templating sections.

The current workaround is to escape `$` with `\$` in Scala (and other) template files to be able to use Scala string interpolation. This needs to be better documented.

There is already #296 and https://github.com/foundweekends/giter8/issues/288#issuecomment-312626433 and #300.

(See also https://github.com/akka/akka-http-quickstart-scala.g8/issues/15)