Add signpost to users of hera

Co-authored-by: Flaviu Vadan <flaviuvadan@gmail.com>
Signed-off-by: Elliot Gunton <egunton@bloomberg.net>

CONTRIBUTING.md

@@ -22,6 +22,7 @@ We also encourage contributions in the form of:
 * Blog Posts / Social Media featuring Hera
 * Attending the [Hera working group meeting](TODO) (bi-weekly on Fridays, 3pm GMT)
   * Add notes to our [community agenda doc](TODO) for the meeting
+  * Adding your organization as a [user of Hera](https://github.com/argoproj-labs/hera/blob/main/USERS.md)
 
 If you have an idea for a large feature, please reach out to us on the Slack channel or attend the working group
 meetings first, and then we can help you propose the feature using