Update CONTRIBUTING.md (mrdoob#9382)

Rich-Harris · mrdoob · commit c3656ca1113e · 2016-07-23T06:47:10.000+09:00
This will hopefully prevent other people making the same mistake as me (mrdoob#9381 (comment))
diff --git a/.github/CONTRIBUTING.md b/.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.
