[docs] Add ShellCheck installation instructions

Summary:
Related Linear issue here. This diff ensures developers install ShellCheck into their dev environments so that it can run with lint-staged on to-be-committed *.sh files.

D4772 adds the rule to lint all to-be-committed *.sh files.

Test Plan:
Tested brew install shellcheck && brew upgrade shellcheck locally on Apple silicon Macbook. On Apple silicon, running which shellcheck should point to /opt/homebrew/bin/shellcheck.

Additionally, created a commit with a problematic *.sh file (after incorporating the changes in D4772) and lint-staged/shellcheck caught the error.

Reviewers: ashoat, varun, jon, atul

Reviewed By: ashoat, varun, jon, atul

Subscribers: tomek, adrian

Differential Revision: https://phab.comm.dev/D4771