docs: add note about shutdown (#642)

Author: ironblaster

README.md

@@ -139,7 +139,6 @@ You can find the latest version at the top of this README or by [searching
 Maven Central](https://search.maven.org/#search%7Cga%7C1%7Ca%3A%22google-maps-services%22) or [Gradle, Please](http://gradleplease.appspot.com/#google-maps-services).
 
 ## Developer Documentation
-
 View the [javadoc](https://www.javadoc.io/doc/com.google.maps/google-maps-services).
 
 Additional documentation for the included web services is available at
@@ -190,6 +189,9 @@ The `GeoApiContext` is designed to be a [Singleton](https://en.wikipedia.org/wik
 in your application. Please instantiate one on application startup, and continue to use it for the
 life of your application. This will enable proper QPS enforcement across all of your requests.
 
+At the end of the execution, call the `shutdown()` method of `GeoApiContext`,
+otherwise the thread will remain instantiated in memory.
+
 For more usage examples, check out [the tests](src/test/java/com/google/maps).
 
 ## Features