Document how to run tests and how to contribute

[ansasaki]

Document in the README (or similar location) instructions on how to run tests and how to contribute.

This should lower the barrier for contributors to learn how to provide tests for their changes.

[kkaarreell]

Hi @ansasaki , we already have
https://github.com/RedHat-SP-Security/keylime-tests/blob/main/TESTING.md
Could you please take a look and maybe point out things that are missing or are too "difficult" to use?
I know there are some parts that are possibly outdated too.

[ansasaki]

I think linking or including the TESTING.md file from the README.md would be sufficient to cover the testing part.

I think it would be good to provide a guide for contribution (instructions on the organization, where to put new tests, and a checklist), maybe in a CONTRIBUTING.md or directly in the README.md.

[kkaarreell]

The TESTING part should be covered by #488