Add stub documenting Spree::ReturnAuthorizations

This commit adds a short article describing RMAs. In the future, it
might make sense to move this to a `../returns` section of the
documentation.

guides/inventory/return-authorizations.md

@@ -0,0 +1,48 @@
+# Return authorizations (RMAs)
+
+After an order is shipped, administrators can approve the return of any part
+of an order in the `solidus_backend` (from the **Orders -> Order -> RMA** page).
+
+Return authorizations are just the first part of the larger returns system built
+into Solidus. When a store administrator creates a new RMA, they fill out a form
+that defines the scope of the anticipated return.
+
+A `Spree::ReturnAuthorization` object has the following attributes:
+
+- `number`: The number assigned to the return authorization. It begins with an
+  `R` and is followed by ten-digits (`RA338330715`).
+- `state`: The state of the return authorization. The state can be `authorized`
+  or `cancelled`. 
+- `order_id`: The ID of the `Spree::Order` associated with this return
+  authorization. 
+- `memo`: An administrative note regarding the authorization.
+- `stock_location_id`: The `Spree::StockLocation` associated with this return
+  authorization.
+- `return_reason_id`: The ID for the `Spree::ReturnReason` associated with this
+  return authorization.
+
+The RMA creation process typically includes the following steps:
+
+1. The administrator presses the **New RMA** button (on the backend's **Orders
+   -> Order -> RMA** page).
+2. They select the items on the order that are being returned.
+3. For each item selected, they choose the following optional values:
+   - They choose a reimbursement type (if applicable). For example: store credit.
+   - They choose an exchange item (if applicable).
+   - They choose a reason for the return authorization for the item (if
+     applicable). For example: "Damaged/Defective".
+4. They choose a stock location that the return is authorized to be shipped to.
+5. They write a memo that documents why the return authorization is being
+   created.  
+6. Once the new RMA form is completed, they press the **Create** button and
+   generate a new `Spree::ReturnAuthorization` object. 
+
+After the customer has mailed their returns back, the administrator can mark the
+the customer return as received (on the backend's **Orders -> Order -> Customer
+Returns** page).
+
+<!-- TODO:
+  Again, we should add links here once additional returns documentation exists
+  here.
+-->
+