title: Create and send cloud messages to Topic in Firebase
body: <p>I am creating an Android application which allows a single user to form a group with all or some of the other users in the app. I am using <code>Firebase</code> for my database. I want to send a push notification to the user when he/she is being added into a group. I can understand how to receive the messages and handle it.</p>
  
  <p>I read the docs over and over again but I have no idea how to actually initiate the group to begin with and how to send push notifications to the users. I have till now only used the <code>Firebase</code> console to send push notifications to all the users in my app.</p>
  
  <p>How can I create a topic such that the users who are in that topic are sent a push notification automatically without using the <code>Firebase</code> console?</p>