Add a YAML syntax hint to dk.rb

Since config.yml needs to be valid YAML, simply adding a line with the path to ruby at the end of it will cause the file to be invalid, resulting in an error when dk.rb install is run. So, this patch includes a hit about YAML syntax in the instructions at the top of the config.yml file.

This tripped me up while installing the devkit. I wasn't sure that I had downloaded the correct version of the devkit, so when I had errors after editing config.yml, I tried the other versions. The problem in the end was that I was adding lines to config.yml with paths in, as suggested by the comment at the top of config.yml, but since I didn't add the array prefix markers the file was failing to parse.