Add more docs for Feature Layer

not complete, but progress..

docs/features/people/employees/index.rst

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+
+Employees
+=========
+
+Employee data normally comes from the POS, if it indeed comes into
+play at all.  You may or may not have a reason to import or otherwise
+act on Employee data using Rattail.
+
+It is the User record in Rattail which is given attribution for
+changes made etc. and not the Employee.  So even if a certain User is
+also an Employee, when logged in and making changes their User account
+is deemed responsible.
+
+Whereas a User record does not technically need to tie back to a
+Person record, an Employee record *must* tie back to a Person.  When
+importing Employee data from POS, both the Person and Employee records
+are created in Rattail.
+
+There are certain places where an Employee *is* assumed, for instance
+the "Buyer" of a Purchase Order will reference an Employee record and
+not a User.  Whereas the creation and execution of a "batch" related
+to purchasing will reference a User.  (See also
+:doc:`../../purchasing/index`.)
+
+Rattail could also be used as a "time clock" system in which case the
+Employee records must obviously be present, for tracking times.
+
+You also can use Rattail to track additional info for each Employee,
+e.g. the start/end dates for their employment over time.  Often an
+employee might come and go more than once, and the POS will rarely
+have a way to track historical dates.  Rattail has a basic way to do
+that built-in, but more "data extensions" are of course possible.