Add initial notes for 1.7.1 (#4026)

Start documenting the changes early.
Signed-off-by: Miek Gieben <miek@miek.nl>

notes/coredns-1.7.1.md

++++
+title = "CoreDNS-1.7.1 Release"
+description = "CoreDNS-1.7.1 Release Notes."
+tags = ["Release", "1.7.1", "Notes"]
+release = "1.7.1"
+date = 2020-06-15T10:00:00+00:00
+author = "coredns"
++++
+
+The CoreDNS team has released
+[CoreDNS-1.7.1](https://github.com/coredns/coredns/releases/tag/v1.7.1).
+
+This is a small, incremental release.
+
+## Brought to You By
+
+
+## Noteworthy Changes
+
+* plugin/forward: register HealthcheckBrokenCount (https://github.com/coredns/coredns/pull/4021).
+  See PR TBD for using `promauto` to never have this 'forget to register metric' again.
+* plugin/trace: Only with *debug* active enable  debug mode for tracing - removes extra logging (https://github.com/coredns/coredns/pull/4016)
+* plugin/grpc: Improve gRPC Plugin when backend is not available (https://github.com/coredns/coredns/pull/3966)
+* project: Add DCO requirement in Contributing guidelines (https://github.com/coredns/coredns/pull/4008)
+* core: Add timeouts for http server (https://github.com/coredns/coredns/pull/3920)