Add slack channel

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/README.md b/README.md
index 8255569..2995c9f 100644
--- a/README.md
+++ b/README.md
@@ -41,4 +41,5 @@
 
 * Our developer mailing list:
   http://lists.runtime.co/mailman/listinfo/dev-mcuboot_lists.runtime.co
+* Our Slack channel: https://runtimeco.slack.com/
 * Our IRC channel: http://irc.freenode.net, #mcuboot