CONTRIBUTING.md - update instructions on linting dependencies

Summary:
While setting up my new computer today I noticed that clang-format-7
is conveniently available in recent Ubuntus, and the recent Debian too.

https://packages.ubuntu.com/search?keywords=clang-format-7
https://packages.debian.org/search?keywords=clang-format-7

Also, I've taken the liberty to add a small note explaining *why* these tools
are needed.

Test Plan: read

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3672