Add "--select E402" to example snippet in `CONTRIBUTING.md` (#6108)
## Summary
In Ruff only a subset of rules are enabled by default. This change
change aims to clarify that when adding a new rule, you must explicitly
use the `--select name_of_rule` command to ensure the rule gets
executed.
This was talked about on Discord a while back.
## Test Plan
Checked docs via mkdocs