Add running tests to the README

[PatrickAlphaC]

I'm not sure how to run tests on this repo. Could we add a test key to the package.json, or at least tell users instructions are in each README.

To add more context, we need to update the CONTRIBUTING.md file entirely. It's not clear how this repo fits together. Is each folder in this repo self-contained? Can I even run the whole test suite across each directory?

The following folders likely need a README.md:

1. l2-contracts
   I was unable to get this test suite to pass.

2. l1-contracts
   I was unable to get this test suite to pass.

3. system-contracts
   The system-contracts folder has a good README.md. I was able to successfully run and execute tests.

With how to run their test suite, and what the folders are intended to do/be.