Add a design/goals document to detail guidelines for the project

[prokopyl]

With the departure of @Janonard as a maintainer, and new contributors coming in, I thought this might be a good time to write down all of my ideas and goals for the lv2 crate(s). This way less time can be spent teaching contributors and users what this crate is and isn't supposed to do.

I created a GOALS.md file detailing all of this (Rendered), and there is also a DESIGN.md file (Rendered) detailing guidelines for API design, although it is still a WIP for now.

As of right now, I do not consider those as final at all. Those are my ideas for now, but any other ideas or discussions about those in the document are welcome to debate and discuss in depth in this PR. :slightly_smiling_face: