[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