Add supply chain security (s3c) working group to list 👷‍♀️

Last week we had the first meeting of a working group focused on cross
project supply chain security interests, this commit adds the details
about this meeting going forward to the list of working groups.

We might want to replace the agenda doc and slack links with something
else if we're not happy with those but this at least gets something in
place :D

working-groups.md

@@ -40,6 +40,7 @@ The current working groups are:
 - [Workflows](#workflows)
 - [Pipeline](#pipeline)
 - [Governing board / Community](#governing-board--community)
+- [Software Supply Chains Security (s3c)](#software-supply-chain-security-s3c)
 
 ## General
 
@@ -385,3 +386,24 @@ and a place to discuss community operations and process.
 | <img width="30px" src="https://github.com/bobcatfish.png"> | Christie Wilson   | Google    | [bobcatfish](https://github.com/bobcatfish) |
 | <img width="30px" src="https://github.com/dibyom.png">     | Dibyo Mukherjee   | Google    | [dibyom](https://github.com/dibyom)         |
 | <img width="30px" src="https://github.com/vdemeester.png"> | Vincent Demeester | Red Hat   | [vdemeester](https://github.com/vdemeester) |
+
+## Software Supply Chain Security (s3c)
+
+The goal of this working group is to discuss supply chain security initiatives across Tekton (exact scope 
+TBD [community#629](https://github.com/tektoncd/community/issues/629)).
+
+| Artifact                   | Link                       |
+| -------------------------- | -------------------------- |
+| Forum                      | [tekton-dev@](https://groups.google.com/forum/#!forum/tekton-dev) |
+| Meeting Notes              | [Discussion board](https://github.com/orgs/tektoncd/projects/14/views/1), [HackMD Notes](https://hackmd.io/gFcAZMFMRwuTaZ1i7Y3fSg)
+| Slack Channel              | [#security](https://tektoncd.slack.com/messages/security) |
+| Community Meeting VC       | [meet.google.com/utk-kdff-hci](https://meet.google.com/utk-kdff-hci) |
+| Community Meeting Calendar | Tuesday every other week, 09:00a-09:30a PST <br>[Calendar](https://calendar.google.com/event?action=TEMPLATE&tmeid=NDFuMjg2OTloYTJrYm1jNGM1dWZiZ3JzdGZfMjAyMjAyMjJUMTcwMDAwWiBjaHJpc3RpZXdpbHNvbkBnb29nbGUuY29t&tmsrc=christiewilson%40google.com&scp=ALL) |
+
+
+| &nbsp;                                                     | Facilitators      | Company   | Profile                                     |
+| --------------------------------------------------------   | ----------        | -------   | ---------------------------------------     |
+| <img width="30px" src="https://github.com/pritidesai.png"> | Priti Desai       | IBM       | [pritidesai](https://github.com/pritidesai) |
+| <img width="30px" src="https://github.com/imjasonh.png">   | Jason Hall        | Red Hat   | [imjasonh](https://github.com/imjasonh)     |
+| <img width="30px" src="https://github.com/bobcatfish.png"> | Christie Wilson   | Google    | [bobcatfish](https://github.com/bobcatfish) |
+| <img width="30px" src="https://github.com/afrittoli.png">  | Andrea Frittoli   | IBM       | [afrittoli](https://github.com/afrittoli)   |