Update merchants.yml

source for merchant addition: https://www.reddit.com/r/Monero/comments/4wbh0t/the_tsunami_is_coming/d6639g2

_data/merchants.yml

@@ -60,6 +60,8 @@
       url: https://cryptostorm.is/
     - name: Esperanto lessons from Kaja
       url: mailto:kiah.morante@gmail.com
+      name: Guitar Music Theory course w/ 30% XMR discount
+      url: http://guitartheoryrevolution.com/
     - name: Web Developer - Stefanos
       url: http://www.stefanosioannou.com/web-development-monero-accepted
     - name: MyMonero Web-based Wallet