README.adoc: add Gitter badge

Let the world know where all the cool people hang out.

README.adoc

@@ -5,6 +5,7 @@ https://travis-ci.org/se-edu/addressbook-level4[image:https://travis-ci.org/se-e
 https://ci.appveyor.com/project/damithc/addressbook-level4[image:https://ci.appveyor.com/api/projects/status/3boko2x2vr5cc3w2?svg=true[Build status]]
 https://coveralls.io/github/se-edu/addressbook-level4?branch=master[image:https://coveralls.io/repos/github/se-edu/addressbook-level4/badge.svg?branch=master[Coverage Status]]
 https://www.codacy.com/app/damith/addressbook-level4?utm_source=github.com&utm_medium=referral&utm_content=se-edu/addressbook-level4&utm_campaign=Badge_Grade[image:https://api.codacy.com/project/badge/Grade/fc0b7775cf7f4fdeaf08776f3d8e364a[Codacy Badge]]
+https://gitter.im/se-edu/Lobby[image:https://badges.gitter.im/se-edu/Lobby.svg[Gitter chat]]
 
 ifdef::env-github[]
 image::docs/images/Ui.png[width="600"]