Add thebigcoin.io to merchants.yml

Fix #406

_data/merchants.yml

@@ -140,6 +140,8 @@
       url: http://dr.mindsci.com/
     - name: Rootbox; Simple, no non-sense hosting
       url: https://rootbox.host/
+    - name: TheBigCoin - Spend Monero Anywhere
+      url: https://www.thebigcoin.io
     - name: Web Developer - Python with Django web framework
       url: http://www.voteforrodneylewis.com
     - name: Web Developer - Stefanos