[DOC] Improve instructions for setting up shellcheck

Summary:
This diff updates the CONTRIBUTING.md to give more info on how to
setup shellcheck from the precompiled binaries, and check the correct
version is in the PATH.

Depends on D7074.

Test Plan: Read the doc and try the instructions.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7075