Add instructions for committers on upgrading cm mode

Author: cldwalker

README.md

@@ -4,7 +4,12 @@ The official JavaScript language plugin for Light Table.
 
 ### For Committers
 
-Upgrade acorn with `npm install acorn`
+* Upgrade acorn with `npm install acorn`
+* Upgrade `javascript.js` with the following where `$VERSION` is a version number tag:
+
+    curl https://raw.githubusercontent.com/codemirror/CodeMirror/$VERSION/mode/javascript/javascript.js > codemirror/javascript.js
+
+  `javascript.js` is currently on CodeMirror version 4.8.0.
 
 ###License