add merchant stamp monero

_data/merchants.yml

@@ -255,6 +255,8 @@
       url: https://silverround.com/
     - name: Synntech Gaming
       url: https://synntechgaming.com
+    - name: Stamp Monero
+      url: https://stampmonero.com/
     - name: Xusah Boutique
       url: https://xusah.boutique/
     - name: Judit Mátyus - Jewelry