updated CONTRIBUTING with instructions for npm run regl-codegen

Author: yujin-wu

CONTRIBUTING.md

@@ -162,6 +162,23 @@ npm run schema
 
 **IMPORTANT:** please do not change and commit any files in the "dist" folder
 
+#### Step 9: REGL - Review & commit potential changes to precompiled regl shaders
+
+If you are implementing a new feature that involves regl shaders, or if you are
+making changes that affect the usage of regl shaders, you would need to run
+
+```bash
+npm run regl-codegen
+```
+
+to regenerate the regl code. This opens a browser window, runs through all
+traces with 'regl' in the tags, and stores the captured code into 
+src/generated/regl-codegen. If no updates are necessary, it would be a no-op, but
+if there are changes, you would need to commit them.
+
+This is needed because regl performs codegen in runtime which breaks CSP
+compliance, and so for strict builds we pre-compile regl codegen now.
+
 #### Other npm scripts that may be of interest in development
 
 - `npm run preprocess`: pre-processes the css and svg source file in js. This