* grep: WIP add exercise

* grep: implement exercise

* grep: add to config.json

* grep: update config.json and add hints

* grep: Add .keep file

* grep: improve reference solution

* grep: Make sure settings.gradle is in alphabetical order

Added new hints.md file copied from exercises/flatten-array/.meta/hints.md
and regenerated the README.md file using configlet. Fixes #1089

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.