Add README for catalog app

openedx/core/djangoapps/catalog/README.rst

+This app serves as an interface to the discovery a.k.a. "catalog" service. It contains
+management commands for caching data from the discovery service, functions for accessing
+that data, and a config model that can be used to control access to the service.
+
+To learn about how programs are cached, please refer to the `the wiki`_.
+
+.. _the wiki: https://openedx.atlassian.net/wiki/display/LEARNER/LMS+Program+Caching