Add release date for v0.20.0

Viktor Lövgren · Viktor Lövgren · commit 1fc60863bae3 · 2019-09-11T12:52:16.000+01:00
diff --git a/notes/0.20.0.markdown b/notes/0.20.0.markdown
@@ -30,3 +30,5 @@
 [#209]: https://github.com/ovotech/fs2-kafka/pull/209
 [#210]: https://github.com/ovotech/fs2-kafka/pull/210
 [#211]: https://github.com/ovotech/fs2-kafka/pull/211
+
+Released on 2019-09-11.
