docs: add basic feature, usage docs

docs/narr/features.rst

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+
+Features
+========
+
+The general idea is to provide an audit/versioning trail for important
+data tables.
+
+Each table defined in the :term:`app model` can either be versioned,
+or not.  Nothing changes for a non-versioned table.
+
+For a "versioned" table, a secondary "versions" table is created,
+schema for which is a superset of the original "versioned" table.
+When records change in the original table, new "version" records are
+added to the versions table.
+
+Therefore you can see how a record has changed over time, by
+inspecting its corresponding versions.
+
+When any record changes (for any versioned table), a new "transaction"
+record is also created.  This identifies the user responsible, and
+timestamp etc.  Any new version records will tie back to this
+transaction record.
+
+All this is made possible by SQLAlchemy-Continuum; the Wutta-Continuum
+package mostly just adds config glue.  See also
+:doc:`sqlalchemy-continuum:index`.