Add SilverRound.com to merchants list.

_data/merchants.yml

@@ -156,6 +156,8 @@
       url: https://synntechgaming.com
     - name: Elise Hawkins Nutritional Therapy  
       url: www.elisehawkinsnutritionaltherapy.com
+    - name: SilverRound.com - Gold and Silver Dealer
+      url: https://silverround.com/
 - category: Entertainment
   merchants:
     - name: MoneroDice