Add contribute.json

This document is required for example for all Mozilla websites
and makes sense for Riot to also describe itself - see
https://www.contributejson.org/

Signed-off-by: Jason Robinson <jasonr@matrix.org>

scripts/copy-res.js

@@ -66,6 +66,7 @@ const COPY_LIST = [
     ["node_modules/matrix-react-sdk/res/media/**", "webapp/media"],
     ["node_modules/olm/olm_legacy.js", "webapp", { directwatch: 1 }],
     ["./config.json", "webapp", { directwatch: 1 }],
+    ["res/contribute.json", "webapp"],
 ];
 
 const parseArgs = require('minimist');