Move contributor list to CONTRIBUTORS.md (#1313)

Created CONTRIBUTORS.md and updated .all-contributorsrc to reference the new file instead of README.md. This will reduce the size of the README and improve loading times.

Author: SableRaf

.all-contributorsrc

@@ -2,7 +2,7 @@
   "projectName": "processing4",
   "projectOwner": "processing",
   "files": [
-    "README.md"
+    "CONTRIBUTORS.md"
   ],
   "imageSize": 120,
   "contributorsPerLine": 6,