improve docs around creating a foreign key constraint

[sbdchd]

Also should note the differences between a new table, and adding to an existing table

https://gocardless.com/blog/zero-downtime-postgres-migrations-the-hard-parts/ https://medium.com/braintree-product-technology/postgresql-at-scale-database-schema-changes-without-downtime-20d3749ed680 https://travisofthenorth.com/blog/2017/2/2/postgres-adding-foreign-keys-with-zero-downtime