doc: add overview of source code for new contributors

[SomeGuyNamedMay]

pretty much what the title says, it would have helped me out a bunch when i started

[trueNAHO]

I suppose the best approach is to add a link in the README.md to a new docs/contributing.md file.

[danth]

We now have a CONTRIBUTING.md, but it's more for organizational things like commit messages and standards.

However, there is a contributors section on the website where we could add this.

[danth]

There are now a couple of additional pages on the website, such as this one. However, there may still be room for improvement here.