fix(contributing.md): added private sign off process (element-hq#26785)

Signed-off-by: Pankaj Singh <[email protected]>

CONTRIBUTING.md

@@ -271,6 +271,17 @@ on Git 2.17+ you can mass signoff using rebase:
 git rebase --signoff origin/develop
 ```
 
+## 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 [email protected]. 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.
+
 # Review expectations
 
 See https://github.com/vector-im/element-meta/wiki/Review-process