Added a hints.md file to the atbash-cipher exercise and generated
the README.md file again using configlet. Fixes #1098

Copied hints.md from flatten-array exercise and
regenerated README.md file via configle.
Fixes #1088

* phone-number add hints
added hints.md file to exercism/java/exercises/phon-number/.meta and generated
readme file again. Fixes #1086

* Added new README.md file

* 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.