added new tutorial to the index of learn page items

Author: aceslowman

src/data/learn/learn.json

@@ -40,6 +40,10 @@
       "name": "3d-webgl",
       "url": "https://github.com/processing/p5.js/wiki/Getting-started-with-WebGL-in-p5"
     },
+    {
+      "name": "shader-intro",
+      "url": "introduction-to-shaders.html"
+    },
     {
       "name": "color",
       "url": "color.html"