* 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.

Add details of README.md to description of submodule.

Add details of how to generate README.md to section on
how to implement new exercise.

Reference solution directory was renamed and move to the
always-excluded-from-delivery .meta folder.

I fixed the link path to end in "installation" instead of "installing"

  I've restructured some things in the docs repository. This fixes the links to the content that moved.