Add extension https://github.com/riknoll/arcade-mml #189

Fixes #189

Author: UnsignedArduino

content/extensions/077-riknoll---arcade-mml.md

@@ -0,0 +1,8 @@
+---
+repo: "https://github.com/riknoll/arcade-mml"
+links:
+  - title: Forum post
+    url: "https://forum.makecode.com/t/extension-arcade-mml/31351?u=unsignedarduino"
+---
+
+Similar to the RTTTL extension that parses RTTTL strings for music, this extension parses [MML](https://en.wikipedia.org/wiki/Music_Macro_Language) strings! This extension adds a `MML` category to the toolbox.