Submitting plugins guidelines

[fath11]

Since Gandi Plugins are still new, we should seriously consider guidelines when someone wants to submit a plugin.
Mainly, we want to make sure plugins that can be merged with another plugin get merged instead of creating an entirely new plugin.

For example, Gandi Beautification plugin can have features such as fully customizable UI color i.e being able to change Gandi's color palette. So when someone wants to implement this feature, they should instead modify Gandi Beautification source code.

I would suggest doing it how Scratch Addons did it https://github.com/ScratchAddons/ScratchAddons

[DilemmaGX]

I think Gandi should consider making a new repo including all guidlines and tips about developing...