title: How to send FCM based on user's location
body: <p>I built a shopping app using React Native. We have a lot of stores and restaurantes from all over the country registered on our app.</p>
  
  <p>We send messages to our users using FCM when a specific store publishes a new offer, based on user interest and the publisher. If a restaurant posted an offer, we will send to all users registered on the topic "/restaurants". This behavior is bad because maybe the restaurant is too far away from the user, and this offer isn't important to him. </p>
  
  <p>That's why I want to create topics based on a location, so if a restaurant from Chicago posts an offer, I would send just to users registered on "/restaurants/chicago". </p>
  
  <p>Is this the best way to do it? Do firebase offer something similar out-of-box? And how would I keep track of users to know when they go from another city from another?</p>