Document how to set up a local development environment

While trying to test PRs in libprocessing, I ran into friction setting up a local development environment for mewnala. It would be helpful to have clear instructions in the README (or in a dedicated CONTRIBUTING.md) explaining how to build and test mewnala locally.

What I ran into was actually an issue specific to #137 but it might still be worth being explicit about how to set up your dev environment as opposed to using the release version.