Add the multi-gitter to to projects_using_cobra.md

Author: lindell

projects_using_cobra.md

@@ -23,6 +23,7 @@
 - [Mattermost-server](https://github.com/mattermost/mattermost-server)
 - [Metal Stack CLI](https://github.com/metal-stack/metalctl)
 - [Moby (former Docker)](https://github.com/moby/moby)
+- [Multi-gitter](https://github.com/lindell/multi-gitter)
 - [Nanobox](https://github.com/nanobox-io/nanobox)/[Nanopack](https://github.com/nanopack)
 - [OpenShift](https://www.openshift.com/)
 - [Ory Hydra](https://github.com/ory/hydra)