Improve doc in readme

Signed-off-by: Louis Chemineau <louis@chmn.me>

README.md

@@ -35,6 +35,7 @@ To enjoy hot module replacement you need two things:
 
 - the `serve` script to your `package.json` as listed above.
 - the [`HMREnabler`](https://github.com/nextcloud/hmr_enabler) server app. This will tweak the CSP header so do not use it in production !
+- add `js/*hot-update.*` to you `.gitignore`. This is necessary because we write every files on disk so the nextcloud server can serve them.
 
 You can then start you dev serve with `npm serve` or `make serve-js` if you added this step in your makefile.