Warn when adding a foreign key?

[ariieh]

We use zero_downtime_migrations at Plated and rely on it pretty heavily for alerting us to dangerous migration situations. However, I recently introduced a PR to our codebase to add a foreign key to a large table we have -- via add_foreign_key -- and was alerted by another developer of ours that this is actually extremely dangerous as it completely locks the table in Postgres:
https://travisofthenorth.com/blog/2017/2/2/postgres-adding-foreign-keys-with-zero-downtime

I was thinking this would be a great candidate for a warning from this gem. Unfortunately there doesn't appear to be a way of doing this more safely (in Rails or SQL), as there is with indices (i.e. algorithm: :concurrently), but I do think at the very least the gem should warn about adding foreign keys and force you to wrap add_foreign_key in safety_assured so you are forced to be aware of this scenario and have to consciously make the decision to go ahead anyway. Happy to introduce a PR to this effect. Thoughts?