Step 1: Creating The Add-On Skeleton gives you the basics and shows how to set up a environment for a SIL add-on.
In Step 2: Creating A New SIL Routine / Step 3: Creating a Custom Field Descriptor we have shown how you should add support for new routines and custom fields.
Now you can use atlas-debug / atlas-run utilities to run the project (with installing the Atlassian SDK is a prerequisite for this).
This page contains the full example, packed in a tar.gz file: example.tar.gz as well as the resulting obr: silexample-4.0.0.obr.
You can use this project as a skeleton to develop your own routines, so have fun, and if you think your routine should stay in core SIL, please email us at products@cprime.com – we're happy to add them.
You can create free plugins based on SIL Engine (former katl-commons) and this documentation but you cannot create commercial (paid) plugins based on SIL. If you wish to do so, contact us at products@cprime.com, and we will discuss the joined approach with you.