Add documentation for revert changes feature.

doc/workflow/revert_changes.md

+# Reverting changes
+
+The new `Revert` button allows you to revert any changes introduced by a Commit or a Merge Request.
+
+## Reverting a Merge Request
+
+After the Merge Request has been merged, a `Revert` button will be available to revert the changes introduced by that Merge Request:
+
+![revert merge request](revert_changes/revert-mr.png)
+
+You can revert the changes directly into the selected branch or you can opt to create a new Merge Request with the revert changes:
+
+![revert merge request modal](revert_changes/revert-mr-modal.png)
+
+After the Merge Request has been reverted, the `Revert` button will not be available anymore.
+
+It's important to mention that this new button will be only available for Merge Requests created since the **8.5** version. However you can still revert a Merge by reverting the merge commit from the list of Commits page.
+
+## Reverting a Commit
+
+You can revert a Commit from the Commit detail page:
+
+![revert commit](revert_changes/revert-commit.png)
+
+In the same way like reverting a Merge Request you can opt to revert the changes directly into the target branch or create a new Merge Request to revert the changes:
+
+![revert commit modal](revert_changes/revert-commit-modal.png)
+
+After the Commit has been reverted, the `Revert` button will not be available anymore.
+
+Please note that when reverting merge commits, the mainline will allways be the first parent, if you want to use a different mainline then you need to do that from the command line, here is a quick sample:
+
+```
+# Revert a merge commit using the second parent as the mainline
+git revert -m 2 commit_hash
+```