Add mention of private sign off to contributing guidelines (#1462)

CONTRIBUTING.rst

@@ -167,3 +167,15 @@ include the line in your commit or pull request comment::
 can't be accepted. Git makes this trivial - just use the -s flag when you do
 ``git commit``, having first set ``user.name`` and ``user.email`` git configs
 (which you should have done anyway :)
+
+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. 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.