Add how to write a breaking change entry to the CONTRIBUTING.md

This PR adds how to write a breaking change entry to the CONTRIBUTING.md

This change is based on:

> All breaking changes are clearly marked with **(Breaking)** in the Changelog.

https://metaredux.com/posts/2019/12/15/a-uniform-rubocop.html

CONTRIBUTING.md

@@ -52,12 +52,14 @@ Here are a few examples:
 ```
 * [#716](https://github.com/rubocop-hq/rubocop/issues/716): Fixed a regression in the auto-correction logic of `MethodDefParentheses`. ([@bbatsov][])
 * New cop `ElseLayout` checks for odd arrangement of code in the `else` branch of a conditional expression. ([@bbatsov][])
+* [#7542](https://github.com/rubocop-hq/rubocop/pull/7542): **(Breaking)** Move `LineLength` cop from `Metrics` department to `Layout` department. ([@koic][])
 ```
 
 * Mark it up in [Markdown syntax][6].
 * The entry line should start with `* ` (an asterisk and a space).
 * 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-hq/rubocop/issues/123): `.
 * Describe the brief of the change. The sentence should end with a punctuation.
+* If this is a breaking change, mark it with `**(Breaking)**`.
 * At the end of the entry, add an implicit link to your GitHub user page as `([@username][])`.
 * If this is your first contribution to RuboCop project, add a link definition for the implicit link to the bottom of the changelog as `[@username]: https://github.com/username`.