Add more docs for Feature Layer

not complete, but progress..

docs/features/people/entry/importing.rst

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+
+Importing
+=========
+
+The most common scenario involves Rattail importing most
+Person-related data from other (e.g. POS or CRM) systems.
+
+However it should be noted that Rattail is meant to live "alongside"
+the other systems, it is not normally going to "replace" them - and
+this means Rattail must *continually* import data from the other
+systems, to remain accurate.  This may be done nightly, or hourly, or
+even in "real-time" (e.g. once every few seconds, or 5 minutes) -
+whatever is needed and/or is possible given the system constraints.
+
+One immediate feature that comes "for free" when importing data into
+Rattail, is its versioning history, aka. audit trail.  Each time an
+import actually *changes* a record in Rattail, the new data is
+separately stored as a "version" record along with the timestamp and
+the "user" who made the change.  In the case of import, the "user" is
+the system from which data is being imported; a dedicated User account
+is created for this purpose (which does not tie back to any Person).
+
+We're talking here of importing data from e.g. POS into Rattail.  But
+if it's also possible to "import" e.g. new Customer records into your
+POS or other system, then Rattail can also be used as a workflow tool
+to assist with organizing new records for import to your other system.
+See :doc:`batches` for more about that.