Added useCallback to README.md, added loading state

README.md

@@ -24,7 +24,15 @@ import useGeolocation from "react-hook-geolocation";
 const ComponentWithGeolocation = () => {
   const geolocation = useGeolocation();
 
-  return !geolocation.error ? (
+  if (geolocation.error) {
+    return <p>No geolocation, sorry.</p>;
+  }
+
+  if (geolocation.isLoading) {
+    return <p>Geolocation data is loading...</p>;
+  }
+
+  return (
     <ul>
       <li>Latitude: {geolocation.latitude}</li>
       <li>Longitude: {geolocation.longitude}</li>
@@ -35,8 +43,6 @@ const ComponentWithGeolocation = () => {
       <li>Speed: {geolocation.speed}</li>
       <li>Timestamp: {geolocation.timestamp}</li>
     </ul>
-  ) : (
-    <p>No geolocation, sorry.</p>
   );
 };
 ```
@@ -59,12 +65,14 @@ const geolocation = useGeolocation({
 
 You can supply a second parameter to `useGeolocation` which will be called every time the data from the Geolocation API is updated. This callback function is then called with the `geolocation` object with all its properties.
 
-If you don't use `PositionOptions`, I recommend that you supply `{}` as your first argument.
+To prevent unnecessary renders you can wrap your callback function in a `useCallback` hook to preserve its referential safety.
+
+If you don’t use `PositionOptions`, I recommend that you supply `{}` as your first argument.
 
 ```jsx
-const onGeolocationUpdate = (geolocation) => {
+const onGeolocationUpdate = useCallback((geolocation) => {
   console.log("Here’s some new data from the Geolocation API: ", geolocation);
-};
+}, []);
 
 const geolocation = useGeolocation({}, onGeolocationUpdate);
 ```