bring back schema isolation on its own page

docs/explanations/schema_isolation.rst

@@ -0,0 +1,15 @@
+.. note::
+
+  This page is a work in progress.
+
+.. _schema_isolation:
+
+Schema Isolation
+================
+
+A PostgREST instance exposes all the tables, views, and stored procedures of a single `PostgreSQL schema <https://www.postgresql.org/docs/current/ddl-schemas.html>`_ (a namespace of database objects). This means private data or implementation details can go inside different private schemas and be invisible to HTTP clients.
+
+It is recommended that you don't expose tables on your API schema. Instead expose views and stored procedures which insulate the internal details from the outside world.
+This allows you to change the internals of your schema and maintain backwards compatibility. It also keeps your code easier to refactor, and provides a natural way to do API versioning.
+
+.. image:: ../_static/db.png