add exchange 'simpleswap' to merchants list

_data/merchants.yml

@@ -51,6 +51,8 @@
       url: https://poloniex.com/exchange/btc_xmr
     - name: ShapeShift (Instant)
       url: https://shapeshift.io/
+    - name: SimpleSwap
+      url: https://simpleswap.io
     - name: Tux Exchange
       url: https://www.tuxexchange.com/trade?coin=XMR&market=BTC
     - name: Trovemat.com (ATM)