Remove old instructions for adding username to CHANGELOG

it is handled automatically now

CONTRIBUTING.md

@@ -60,7 +60,7 @@ Here are a few examples:
 * If the change has a related GitHub issue (e.g. a bug fix for a reported issue), put a link to the issue as `[#123](https://github.com/rubocop/rubocop-minitest/issues/123): `.
 * Describe the brief of the change. The sentence should end with a punctuation.
 * At the end of the entry, add an implicit link to your GitHub user page as `([@username][])`.
-* Alternatively, you may modify the CHANGELOG file directly, but this may result in conflicts later on. Also, if this is your first contribution to RuboCop Minitest project, add a link definition for the implicit link to the bottom of the changelog as `[@username]: https://github.com/username`.
+* Alternatively, you may modify the CHANGELOG file directly, but this may result in conflicts later on.
 
 [1]: https://github.com/rubocop/rubocop-minitest/issues
 [2]: https://www.gun.io/blog/how-to-github-fork-branch-and-pull-request