Add hints to exercises without starter implementation (#1084)
* Add hints to exercises without starter implementation
As exercises with difficulty 5 are the first exercises without starter
implementation, they should have a hints file explaining how to fix the
compiler errors that occur when an exercise doesn't have any starter
implementation.
This should be detailed in the policies doc.
* flatten-array: regenerate README
Also update description for how to generate readme as it was lacking
detail.
Replace exercise templates with track-level default
You can still override the track-level default template by adding a .meta/readme.go.tmpl for the exercise in question.
This removes the stray newline at the end of the generated READMEs.