Add external-dns README

internal/external-dns/README.md

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+# ExternalDNS
+
+To ease development, code reviews, and make it easier for developers to get up to speed with plan/registry/provider code, we will initially work off a copy of the relevant external-dns code in this repo. When we have a solution that works for us we will look into how we can submit that back to external-dns.
+
+Code is copied, unmodified where possible, from the v0.14.0 version of external-dns. https://github.com/kubernetes-sigs/external-dns/tree/v0.14.0
+
+If you are updating anything from the external-dns repo make sure you use this version, and keep all changes being made to external-dns code in their own commit and mark as "external-dns" in the comments.