Update CONTRIBUTING.md

Author: dsp-ant

CONTRIBUTING.md

@@ -29,6 +29,8 @@ npm install  # install dependencies
 ```
 
 ## Making Changes
+Note that schema changes are made to `schema.ts`. `schema.json` is generated from `schema.ts`
+using `npm run validate:schema`.
 
 1. Create a new branch:
 ```bash