message: contributing: DCO fixup best practice.
diff: @@ -167,7 +167,24 @@ You can add the sign off when creating the git commit via `git commit -s`.
  If you want this to be automatic you can set up some aliases:
  -```
  +```bash
  git config --add alias.amend "commit -s --amend"
  git config --add alias.c "commit -s"
  ```
  +
  +## Fixing DCO
  +
  +If your PR fails the DCO check, it's necessary to fix the entire commit history in the PR. Best
  +practice is to [squash](http://gitready.com/advanced/2009/02/10/squashing-commits-with-rebase.html)
  +the commit history to a single commit, append the DCO sign-off as described above, and [force
  +push](https://git-scm.com/docs/git-push#git-push---force). For example, if you have 2 commits in
  +your history:
  +
  +```bash
  +git rebase -i HEAD^^
  +(interactive squash + DCO append)
  +git push origin -f
  +```
  +
  +Note, that in general rewriting history in this way is a hinderance to the review process and this
  +should only be done to correct a DCO mistake.
  