Fix broken link in docs/README.md
## Quick start
-- [Getting started](getting-started.md)
+- [Cluster API Quick Start](https://cluster-api.sigs.k8s.io/user/quick-start.html)
- [List of AMIs](amis.md)
- [Accessing cluster instances](accessing-instances.md)
- [Building AMIs with Packer](https://github.com/kubernetes-sigs/image-builder/tree/master/images/capi#make-targets)
