adding indexes on tables in migration scripts

I have seen several developers new (and old) to rails miss adding indexes to columns in migration scripts.  In the vast majority of use cases adding indexes to foreign keys is best practice. As this guide is a top search result I feel that it's important to show best practices rather than a minimum working solution.

With so much magic in Rails, one may expect indexes to be added for you.  Having them in the guide serves as a reminder that you need to explicitly add them yourself.
And in all your additions.