Update CONTRIBUTING.md

Signed-off-by: Max Wu <[email protected]>

Author: jackycute

CONTRIBUTING.md

@@ -71,3 +71,5 @@ If you set your `user.name` and `user.email` git configs, you can sign your
 commit automatically with `git commit -s`. You can also use git [aliases](https://git-scm.com/book/tr/v2/Git-Basics-Git-Aliases)
 like `git config --global alias.ci 'commit -s'`. Now you can commit with
 `git ci` and the commit will be signed.
+
+[dcofile]: https://github.com/hackmdio/codimd/blob/develop/contribute/developer-certificate-of-origin