Add ShapeShift to exchanges.

_data/merchants.yml

@@ -4,6 +4,8 @@
       url: https://www.bittrex.com/Market/Index?MarketName=BTC-XMR
     - name: Poloniex
       url: https://poloniex.com/exchange/btc_xmr
+    - name: ShapeShift (instant)
+      url: https://shapeshift.io/
 - category: Block Explorers
   merchants:
     - name: ChainRadar