add documentation for how to enable source completion in zsh

README.md

@@ -70,11 +70,20 @@ sudo mv bin/darwin-amd64/gardenctl-darwin-amd64 /usr/local/bin/gardenctl
 
 `gardenctl` supports auto completion. This recommended feature is bound to `gardenctl` or the alias `g`. To configure it you can run:
 
+if you are using `bash`:
+
 ```bash
 echo "source <(gardenctl completion bash)" >> ~/.bashrc
 source ~/.bashrc
 ```
 
+if you are using `zsh`:
+
+```bash
+echo "source <(gardenctl completion zsh)" >> ~/.zshrc
+source ~/.zshrc
+```
+
 ### Via Dockerfile
 
 First clone the repository as described in the the build step "From source". As next step add the garden "config" file and "clusters" folder with the corresponding kubeconfig files for the garden cluster. Then build the container image via `docker build -t gardener/gardenctl:v1 .` in the cloned repository and run a shell in the image with `docker run -it gardener/gardenctl:v1 /bin/bash`.