update README for MacOS 10.13.4 High Sierra

Author: szTheory

README.md

@@ -1,5 +1,5 @@
 # SimpleSynth - A MIDI synth for Mac
 
-I added a volume control slider and updated the project to build for El Capitan.
+I added a volume control slider and updated the project to build for MacOS 10.13.4 High Sierra.
 
 ![38577945-715d8320-3cd0-11e8-801e-3fa0b0b9b03e.png](https://user-images.githubusercontent.com/28652/38577945-715d8320-3cd0-11e8-801e-3fa0b0b9b03e.png)