Add Flutter Shared Instance Documentation

This documents the new create and get instance methods.

docs/flutter/usage.md

@@ -46,3 +46,38 @@ appfit.identifyUser(null);
 # Cached Events
 
 We cache all event locally in the SDK, allowing us to retry failed events. If a device is experiencing network issues, we will retry events once the device is back online to help avoid event data loss.
+
+# Shared Instances
+
+Alternativly, we support using shared instances. This can be achieved by calling two methods.
+
+`createInstance` and `getInstance`
+
+## Creating a Shared Instance
+
+To create a shared instance that can be shared is as simple as using the static method on the `AppFit` class.
+
+```dart
+// Create the configuration
+final configuration = AppFitConfiguration(apiKey: 'API_KEY');
+
+// Create the shared instance
+AppFit.createInstance(configuration)
+```
+
+This will create an instance called `default`. Alternativly you can pass in a parameter `instanceName` if you are running multiple instances.
+
+Note: The `createInstance` will return the newly created instance if you need access to it right away for tracking purposes.
+
+### Access a Shared Instance
+
+When you need to access the shared instance to perform a tracking event, you can call the `getInstance` method.
+
+```dart
+// Return an instance
+AppFit.getInstance()
+```
+
+This will return an instance called `default`. Alternativly you can pass in a parameter `instanceName` if you are running multiple instances.
+
+Note: If you call this method before creating an instance, an exception will be thrown.