In this page you can find the API reference and tutorials about how to write a plugin.
The API is available under the CC BY-ND 4.0 license.
Here are some tutorials that will help you to understand how to write Marta plugins.
Swift API was deprecated in favour of the Lua API and will be removed in future versions of Marta. Lua API is simpler, it allows to maintain backwards compatibility, and it does not require installing Xcode. See the Lua API blog post for more information.
The Swift API is still available in Marta 0.7, and you are free to use it. However, if you do, please let me know why you decided not to migrate to the Lua API.