explain how to test local changes

docs/CONTRIBUTING.en.md

@@ -28,3 +28,30 @@ Thank you for your PR! Before creating a PR, please check the following:
 
 Thanks for your cooperation 🤗
 
+## Testing with Sharkey
+
+Let's say you have Sharkey checked out at `~/src/Sharkey`, and this
+repository at `~/src/sfm-js`. You have made some modifications to this
+code, and want to run Sharkey with your modifications.
+
+```shell
+cd ~/src/Sharkey
+pnpm --filter=backend add ../../../sfm-js
+pnpm --filter=frontend add ../../../sfm-js
+```
+
+this will replace the "real" `@transfem-org/sfm-js` in the
+dependencies, with your local version. Remember to *not* commit the
+changes to `package.json` that this causes! (`pnpm link` should also
+work, but I couldn't figure it out)
+
+Then you'll need to tell Vite to handle that package specially. Edit
+`packages/frontend/vite.config.local-dev.ts` and add, before `define`:
+
+```json
+	optimizeDeps: {
+		include: ['@transfem-org/sfm-js'],
+	},
+```
+
+Now you can `pnpm dev` and test your changes.