Marketplace: Lay out the HCI for Shopper Purchasing Products

#831

From: #1057 (comment)

app/furniture/marketplace/README.md

@@ -1,5 +1,18 @@
 # Marketplace
 
+## Human/Computer/Human Interaction Design
+
+### Shopper Purchases Products
+
+<!-- it would be cool if someone could Mermaid this -->
+
+1. `Shopper` links `Product`s to their `Cart` by creating a `CartProduct` record which keeps track of things like quantity, discounts, special requests, etc.
+2. `Shopper` creates a `Checkout`, which directs them to the `Distributor`'s Payment Processor (Stripe, for now).
+3. `Shopper` completes the Payment Processor's flow, which directs them back to Convene and updates the `Checkout` with payment processor details.
+4. `Shopper` is redirected to the `Order`, which represents the `Cart` to the `Shopper` in a read-only manner.
+
+## Architecture
+
 The Marketplace uses Stripe, we anticipate that we will want to use the "Connect then Transfer" workflow: https://stripe.com/docs/connect/collect-then-transfer-guide
 
 1. Build the Workflow for connecting a Stripe Account