Add basic schema docs

docs/concepts/schema.rst

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+
+Database Schema
+===============
+
+.. contents:: :local:
+
+Rattail provides a "core" schema which is assumed to be the foundation of any
+Poser app database.
+
+
+Core Tables
+-----------
+
+All tables which are considered part of the Rattail "core" schema, are defined
+as ORM classes within the ``rattail.db.model`` package.
+
+.. note::
+
+   The Rattail project has its roots in retail grocery-type stores, and its
+   schema reflects that to a large degree.  In practice however the software
+   may be used to support a wide variety of apps.  The next section describes
+   that a bit more.
+
+
+Customizing the Schema
+----------------------
+
+Almost certainly a custom app will need some of the core tables, but just as
+certainly, it will *not* need others.  And to make things even more
+interesting, it may need some tables but also need to "supplement" them
+somehow, to track additional data for each record etc.
+
+Any table in the core schema which is *not* needed, may simply be ignored,
+i.e. hidden from the app UI etc.
+
+Any table which is "missing" from core schema, from the custom app's
+perspective, should be added as a custom table.
+
+Also, any table which is "present but missing columns" from the app's
+perspective, will require a custom table.  In this case each record in the
+custom table will "tie back to" the core table record.  The custom record will
+then supply any additional data for the core record.
+
+Defining custom tables, and associated tasks, are documented in
+:doc:`../schemachange`.