We should add Tor Messenger to the "Which PGP keys sign which packages" page

We should add Sukhbir's key as the key that sign Tor Messenger packages to this page: https://www.torproject.org/docs/signing-keys.html.en

pub   4096R/6887935AB297B391 2011-06-06 [expires: 2016-06-04]
      Key fingerprint = 3A0B 3D84 3708 9613 6B84  5E82 6887 935A B297 B391
uid                          Sukhbir Singh <sukhbir.in@gmail.com>
uid                          Sukhbir Singh <azadi@debian.org>
uid                          Sukhbir Singh <azadi@riseup.net>
uid                          Sukhbir Singh <s3singh@uwaterloo.ca>
uid                          Sukhbir Singh <sukhbir@torproject.org>
uid                          Sukhbir Singh <sukhbir.singh@uwaterloo.ca>
sub   4096R/87C9D96A0C8CEA2A 2011-06-06 [expires: 2016-06-04]