Add PayPal article to resource (#50)

Author: mristau-wm

README.md

@@ -259,3 +259,4 @@ Interesting reads:
 - [Safe Operations For High Volume PostgreSQL](https://www.braintreepayments.com/blog/safe-operations-for-high-volume-postgresql/)
 - [Rails Migrations with Zero Downtime](https://blog.codeship.com/rails-migrations-zero-downtime/)
 - [Stop worrying about PostgreSQL locks in your Rails migrations](https://medium.com/doctolib/stop-worrying-about-postgresql-locks-in-your-rails-migrations-3426027e9cc9)
+- [PostgreSQL at Scale: Database Schema Changes Without Downtime](https://medium.com/paypal-tech/postgresql-at-scale-database-schema-changes-without-downtime-20d3749ed680)