update doc for package.json linting

Issue: CLDSRV-875

Author: benzekrimaha

CONTRIBUTING.md

@@ -15,3 +15,13 @@ yarn dev
 
 This will start the developement server, watching for changes and restarting
 as needed.
+
+### Formatting
+
+Prettier is enforced in CI for changed files, including `package.json`.
+Since `yarn add`/`yarn remove` may rewrite `package.json` with 2-space
+indentation, run formatting before pushing dependency changes:
+
+```bash
+yarn format
+```