Update CONTRIBUTING.md to clang-7

Summary: Also add apt package link

Test Plan: I used the apt.llvm.org source to install clang-7 in Ubuntu, and it worked.

Reviewers: Fabien, deadalnix, jasonbcox, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: markblundeberg, teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2343