Added all-contributors shield to README.md (commented out for now)

Author: SableRaf

README.md

@@ -1,3 +1,6 @@
+<!-- Uncomment the shield below once the repo is made public -->
+<!-- [![All Contributors](https://img.shields.io/github/all-contributors/processing/processing4?color=ee8449)](#contributors) -->
+
 <img src="https://processing.org/favicon.svg" width="250">
 
 Processing is a flexible software sketchbook and a programming language designed for learning how to code.