Added guide for OSX users on how to install clang-format-8.

Summary: Added the command that allowed me to use clang-format-8 without having to recompile llvm & clang to CONTRIBUTING.md.

Test Plan: I ran npm install -g clang-format@1.2.4. I had to delete/move some files due to conflicts with another installation, but after that, running arc diff succeeded. A fresh installation should work with just that command; otherwise the instructions can be amended to accomodate for additional problems.

Reviewers: deadalnix, O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4862