SCRIPTS: add the slack channel URL to the announce script

It's just to provide the URL in the usual URLs when releasing.
diff --git a/scripts/announce-release b/scripts/announce-release
index c4045ae..90a36c0 100755
--- a/scripts/announce-release
+++ b/scripts/announce-release
@@ -155,6 +155,7 @@
 (echo "Please find the usual URLs below :"
  echo "   Site index       : http://www.haproxy.org/"
  echo "   Discourse        : http://discourse.haproxy.org/"
+ echo "   Slack channel    : https://slack.haproxy.org/"
  echo "   Sources          : http://www.haproxy.org/download/${BRANCH}/src/"
  echo "   Git repository   : http://git.haproxy.org/git/${gitdir}/"
  echo "   Git Web browsing : http://git.haproxy.org/?p=${gitdir}"