Add CONTRIBUTING file

[fcalderon]

Hi there, thanks so much for this project, I love these toasts.

I noticed that there isn't a CONTRIBUTING file. If contributors are welcomed to suggest changes via PRs, can you please add a CONTRIBUTING file? This file might even be useful if contributions are not welcome!

Thanks!

[Stabzs]

This is a great suggestion and while it isn't one that is necessarily a priority, it certainly would add value to the project. This is something I will try to get to.

[HZooly]

Hello @Stabzs, any progression on it ? 🤗