Sometimes, a database admin has to look up audits by IP address for the Policy & Abuse committee. Audits aren't currently indexed on IP address (remote_address). Doing so would make this a bit faster.
(This table is managed by a gem, but adding an extra index shouldn't interfere with how it works.)
Because audits is a hefty table, the migration should use pt-online-schema-change in staging/production.
To test, run the migration on staging and make sure it's successful.