Suggested updates to contribution guidelines

[dominics-studio]

Please consider noting the Extension system as an alternative to contributing pull requests in the contribution guidelines, perhaps with example use cases (it's hard for me to gauge what is and isn't possible with extensions from reading the docs).

I read the guidelines then spent a day or so preparing a pull request for a new UI panel when I stumbled across the Extensions system. I didn't migrate my fork to an extension, because I already have a working build! And now I'm not so comfortable raising a PR because my use case is so specific it should have been an extension. I wish I knew about extensions before starting the work.

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context: I wanted a user_data editing panel to help me storyboard

https://github.com/dominics-studio/Pixelorama/tree/userdata-editor-panel