Note that contributors can sign off privately (#12204)

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>

changelog.d/12204.doc

+Document that contributors can sign off privately by email.

docs/development/contributing_guide.md

@@ -458,6 +458,17 @@ Git allows you to add this signoff automatically when using the `-s`
 flag to `git commit`, which uses the name and email set in your
 `user.name` and `user.email` git configs.
 
+### Private Sign off
+
+If you would like to provide your legal name privately to the Matrix.org
+Foundation (instead of in a public commit or comment), you can do so
+by emailing your legal name and a link to the pull request to
+[dco@matrix.org](mailto:dco@matrix.org?subject=Private%20sign%20off).
+It helps to include "sign off" or similar in the subject line. You will then
+be instructed further.
+
+Once private sign off is complete, doing so for future contributions will not
+be required.
 
 # 10. Turn feedback into better code.