Update CONTRIBUTING.md (#9382)

This will hopefully prevent other people making the same mistake as me (https://github.com/mrdoob/three.js/pull/9381#issuecomment-234625456)

.github/CONTRIBUTING.md

@@ -25,4 +25,4 @@
 2. Fork the repository on GitHub.
 3. Check the [Contribution Guidelines](https://github.com/mrdoob/three.js/wiki/How-to-contribute-to-three.js).
 4. Make changes to your clone of the repository.
-5. Submit a pull request.
+5. Submit a pull request. Don't include build files in the PR.