Add #garden and #garden-dev Slack Channels

Hi! I work at [Garden.io](https://garden.io/), we're responsible for the eponymous Kubernetes development tool, [Garden](https://github.com/garden-io/garden). 

We'd like to be more easily accessible to the Kubernetes community, and since Garden is 100% OSS, we'd like to have our development channel on Kubernetes Slack as well.

communication/slack-config/channels.yaml

@@ -61,6 +61,8 @@ channels:
   - name: fi-users
   - name: fr-events
   - name: fr-users
+  - name: garden
+  - name: garden-dev
   - name: gardener
   - name: gimbal
   - name: github