docs(contributing): add note about order of entries

CONTRIBUTING.md

@@ -65,13 +65,16 @@ A new pixel has to be an object with the following four properties:
 - `color`: The color your pixel should have as a hex code (e.g. #ff0000 for red)
 - `username`: The GitHub username you'll use to create the pull request
 
+The row for your pixel should be sorted by the y-coordinate first and then by the x-coordinate. If you are unsure about your changes, make the change and run `npm run format:json` and it should sort your pixel into the appropriate position.
+
 The change should look like this:
 
 ```diff
 {
   "data": [
-+    { "y": 10, "x": 20, "color": "#FFFF00", "username": "dkundel" },
-    { "y": 1, "x": 3, "color": "#F22F46", "username": "<UNCLAIMED>" },
+    {"y": 1, "x": 3, "color": "#F22F46", "username": "twilio-labs"},
++    { "y": 1, "x": 4, "color": "#FFFF00", "username": "dkundel"},
+    {"y": 2, "x": 9, "color": "#F22F46", "username": "twilio"},
     ...
   ]
 }