add a link to documentation

README.md

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 ExternalDNS synchronizes exposed Kubernetes Services and Ingresses with DNS providers.
 
+## Documentation
+
+This README is a part of the complete documentation, available [here](https://kubernetes-sigs.github.io/external-dns/).
+
 ## What It Does
 
 Inspired by [Kubernetes DNS](https://github.com/kubernetes/dns), Kubernetes' cluster-internal DNS server, ExternalDNS makes Kubernetes resources discoverable via public DNS servers. Like KubeDNS, it retrieves a list of resources (Services, Ingresses, etc.) from the [Kubernetes API](https://kubernetes.io/docs/api/) to determine a desired list of DNS records. *Unlike* KubeDNS, however, it's not a DNS server itself, but merely configures other DNS providers accordingly—e.g. [AWS Route 53](https://aws.amazon.com/route53/) or [Google Cloud DNS](https://cloud.google.com/dns/docs/).