Nannou: Add section about how to contribute (to the guide)

[madskjeldgaard]

Would be nice with a section about how to contribute to the guide, including the git workflow and how rustbook works in this case, explaining the compilation of the code examples in the build process.

[mitchmindtree]

I really like the sound of this!

Perhaps we can add a "Contributing" chapter between the "Tutorials" and "Developer Reference" chapters? This way we could lead on nicely from "how to contribute" to "how to navigate nannou's code-base". What are your thoughts? Let me know if you had something else in mind!

On a related note, I think there's a way you can provide a "template" PR on github where the PR itself gives you a checklist of things to check. Would be nice to add this too!

I think github also has support for a CONTRIBUTING.md, so perhaps after this issues is addressed we could create one that points directly to the "Contributing" chapter within the guide.

[mitchmindtree]

Some ideas discussed with @sbauer322:

• A documentation style/format for API docs.
• Documentation goals for public API, e.g. examples, input/output ranges, edge cases.