The package needs to be provided to the ReactInstanceManager when it is built. To accomplish this you will need to insert an .addPackage(new YourPackageName()) call to the mReactInstanceManager = ReactInstanceManager.builder() call chain.
Refer to the code below and add the addPackage statement to your application's MainActivity.java file. This file exists under the android folder in your react-native application directory. The path to this file is: android/app/src/main/java/com/your-app-name/MainActivity.java.
To make it simpler to access your new functionality from JavaScript, it is common to wrap the native module in a JavaScript module. This is not necessary but saves the consumers of your library the need to pull it off of NativeModules each time. This JavaScript file also becomes a good location for you to add any JavaScript side functionality.
The package needs to be provided in the getPackages method of the MainActivity.java file. This file exists under the android folder in your react-native application directory. The path to this file is: android/app/src/main/java/com/your-app-name/MainActivity.java.
To make it simpler to access your new functionality from JavaScript, it is common to wrap the native module in a JavaScript module. This is not necessary but saves the consumers of your library the need to pull it off of NativeModules each time. This JavaScript file also becomes a good location for you to add any JavaScript side functionality.