Add license information to properties and guidance on licensing in documentation

[SableRaf]

Context

A lot of existing Processing libraries don’t have an explicit license, and we haven’t really pushed for using open-source licenses in the past.

Yet, third-party libraries are often where new features get tried out before they make it into core. For this to be possible, they need to be using a compatible license.

Specifically, it would be a good idea for us to recommend using the LGPL v2.1 license which Processign core uses. This makes it easier to eventually bring features into core.

Note: Using GPL can be problematic because, while you can use LGPL code in a GPL project, you can’t do it the other way around.

Other permissive licenses like MIT are also compatible, but they’re less aligned with the spirit of the project since they allow commercial use of your code without contributing improvements back to the community.

Suggestion

It would make sense to add license info to the library properties and include some basic guidance on licensing in the documentation.

[mingness]

This idea sounds good to me!