Add EDDM size, income, businesses and USPS map link (#13)

## Summary
Closes #13.
- Mirror Monica: join USPS EDDM GIS stats onto the carrier-route picker and link to the official EDDM map.
- GIS miss stays a dash and does not block Find routes; PCM households/cost unchanged.

site/directmail/README.md

@@ -106,6 +106,11 @@ Route mode calls PCM `POST /list/count/zipcode` (or `/radius`) with
 or `PCM_POSTCARD_PRICE_FIRSTCLASS` / `PCM_POSTCARD_PRICE_STANDARD`. Default
 purchased list type is `IRL` (`PCM_LIST_TYPE`). This is not official EDDM postage.
 
+Compose also joins USPS EDDM GIS stats (businesses; household size / median
+income when USPS fills them) and links to
+[the EDDM map](https://eddm.usps.com/eddm/select-routes.htm) for the official
+route table.
+
 ### Campaign completion email
 
 When a campaign reaches **completed** (email, SMS, or postcard), one summary email