docs: Add prerequisites for regenerating dist with ncc

Author: bbhtt

CONTRIBUTING.md

@@ -6,10 +6,14 @@ If you plan to contribute to flatpak-github-actions, here's a couple of things t
 
 - NodeJS 16.x or newer
 - Yarn
-- `@vercel/ncc` you can install it with `yarn global add @vercel/ncc`
 
 For more details, we recommend looking the extensive guide at [Creating a JavaScript action](https://docs.github.com/en/actions/creating-actions/creating-a-javascript-action#prerequisites)
 
+```shell
+yarn global add @vercel/ncc
+yarn --cwd flatpak-builder --frozen-lockfile && yarn --cwd flat-manager --frozen-lockfile
+```
+
 Once you have modified the `index.js` of either `flatpak-builder` or `flat-manager` action. Make sure to compile the file to the `dist` directory. You can do so with
 
 ```shell