added requirement for sha1sum on Mac
added requirement for sha1sum on Mac

When running on Mac if you don't have sha1sum available the following error will happen:

```
Unable to find image 'busybox:1.26.2' locally
1.26.2: Pulling from library/busybox
27144aa8f1b9: Pull complete 
Digest: sha256:be3c11fdba7cfe299214e46edc642e09514dbb9bbefcd0d3836c05a1e0cd0642
Status: Downloaded newer image for busybox:1.26.2
./dind-cluster-v1.6.sh: line 322: sha1sum: command not found
```

This comment adds the requirement to the README
Mirantis/kubeadm-dind-cluster