Add release note for v1.6.6 (#3531)

* Add release note for v1.6.6
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Update release note to cover review comments
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

notes/coredns-1.6.6.md

++++
+title = "CoreDNS-1.6.6 Release"
+description = "CoreDNS-1.6.6 Release Notes."
+tags = ["Release", "1.6.6", "Notes"]
+release = "1.6.6"
+date = 2019-12-11T10:00:00+00:00
+author = "coredns"
++++
+
+The CoreDNS team has released
+[CoreDNS-1.6.6](https://github.com/coredns/coredns/releases/tag/v1.6.6).
+
+A fairly small release that polishes various plugins and fixes a bunch of bugs.
+
+
+# Security 
+
+The github.com/miekg/dns has been updated to v1.1.25 to fix a DNS related security
+vulnerability (https://github.com/miekg/dns/issues/1043).
+
+# Plugins
+
+A new plugin [*bufsize*](/plugins/bufsize) has been added that prevents IP fragmentation
+for the DNS Flag Day 2020 and to deal with DNS vulnerabilities.
+
+* [*cache*](/plugin/cache) added a `serve_stale` option similar to `unbound`'s `serve_expired`.
+* [*sign*](/plugin/sign) fix signing of authoritative data that we are not authoritative for.
+* [*transfer*](/plugin/transfer) fixed calling wg.Add in main goroutine to avoid race conditons.
+
+## Brought to You By
+
+Chris O'Haver
+Gonzalo Paniagua Javier
+Guangming Wang
+Kohei Yoshida
+Miciah Dashiel Butler Masters
+Miek Gieben
+Yong Tang
+Zou Nengren
+
+## Noteworthy Changes
+
+* plugin/bufsize: A new bufsize plugin to prevent IP fragmentation and DNS Flag Day 2020 (https://github.com/coredns/coredns/pull/3401)
+* plugin/transfer: Fixed calling wg.Add in main goroutine to avoid race conditions (https://github.com/coredns/coredns/pull/3433)
+* plugin/pprof: Fixed a reloading issue (https://github.com/coredns/coredns/pull/3454)
+* plugin/health: Fixed a reloading issue (https://github.com/coredns/coredns/pull/3473)
+* plugin/redy: Fixed a reloading issue (https://github.com/coredns/coredns/pull/3473)
+* plugin/cache: Added a `serve_stale` option similar to `unbound`'s `serve_expired` (https://github.com/coredns/coredns/pull/3468)
+* plugin/sign: Fix signing of authoritative data (https://github.com/coredns/coredns/pull/3479)
+* pkg/reuseport: Move the core server listening functions to a new package (https://github.com/coredns/coredns/pull/3455)