Add basic overview of Person-related model structure

docs/features/people/index.rst

@@ -6,6 +6,40 @@ Rattail considers the Person to be a central concept, which underpins
 various other concepts such as User, Customer etc. and ideally ties
 them together (e.g. one Person may be both a Customer and Employee).
 
+It can be tricky to keep straight all the various relationships
+between e.g. a Customer and Person.  Here is a quick overview of the
+*intended* structure:
+
+* **Person** - represents exactly one (real) human
+
+* **User** - represents a user account within the Rattail system.
+  typically a user associates with one person, but "system" users do
+  not
+
+* **Employee** - represents an employee in the org; this is always
+  associated with one person
+
+* **Customer** - represents a customer account for the org.  this may
+  have an "account holder" set, which is a person.  it also may have
+  shoppers (next)
+
+* **CustomerShopper** - represents a "shopper" for a customer account.
+  these always relate not only to a customer, but also (separately) to
+  a person who "is" the shopper.  the first shopper is usually the
+  account holder, and is treated as such if no account holder is set
+
+* **CustomerPerson** - DEPRECATED but documented here for clarity.
+  there is a table just for connecting person and customer accounts.
+  it is no longer needed since the customer now has "account holder"
+  and "shoppers" which are more explicit
+
+* **Member** - represents a member account for the org.  in a retail
+  context this usually associates with a customer account.  it
+  probably also has an "account holder" which is a person
+
+* **VendorContact** - represents a contact for a vendor.  always
+  associates with one vendor and one person
+
 Rattail can import this data from other (e.g. POS) systems, but it
 also can *export* data to other systems.  And once this data is in
 Rattail it can be used for other features as needed; for instance