Once you have written an add-in, you can upload it to Pinta's add-in repository and make it available for other Pinta users to download and install.
You will need the mautil program from Mono.Addins (mono-addins-utils in your package manager). Run
mautil pack on your addin DLL file to generate an mpack file. This file format is understood by Pinta's addin manager, and you can test that you have built it correclty by making a local install of the mpack file from the add-in manager.
Clone the Pinta Community Addins repository, and add the mpack file there according to the instructions. Then submit a pull request and when it is accepted, your add-in will be magically available to Pinta users worldwide! (Provided they're on 1.6 or later or have added the repository themselves.)
If you're using Pinta 1.5 and want some add-in automagical goodness, add this url to your Add-Manager (Manage repositories -> Add)