Add first docs for Rattail DB, and POS / other data stores

docs/data/db/extend.rst

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+
+Extending the Schema
+====================
+
+While the core Rattail schema provides "most" tables you may need, it is quite
+common for an app to need yet more tables to satisfy its features.
+
+Extending the schema really just means, you can add extra tables as needed, and
+modify the schema for each of those going forward.  The core tables provided by
+Rattail schema should never be directly modified, but your extra tables are
+fair game.
+
+It is important that you name all extra tables with a prefix unique to your
+app, e.g. ``poser_product`` for a product extension table, or ``poser_widget``
+for a totally custom table.  Avoid prefix-less names like ``widget`` because
+Rattail reserves the prefix-less namespace for its core schema.  Also, certain
+integration packages reserve other namespaces (e.g. ``corepos_`` prefix is used
+by the rattail-corepos package for extension tables it provides).
+
+TODO: obviously need to say how one can go about extending schema...