Add CONTRIBUTING.md

[KristijanArmeni]

We should add some basic guidelines on how to contribute a new test. The goal is to make it easier for new users to contribute their test and what is expected.

For now that can be a "How to contribute" section in the README.md or a standalone CONTRIBUTING.md file. Minimally, it should provide the following:

• where to put the code for a test
• what files must be in the folder (e.g. interface.py, main.py)
• what each of the files mean and what contents (functions, docs etc.) should the new contributor put there
• How to contribute once the test is written (via a pull request)

In general, the Participation guidelines section of The Turing Way book gives a nice overview of how good practices look like.

[soul-codes]

@KristijanArmeni I was thinking a bit about this. Ideally, we want the best of both worlds:

1. The entry barrier for new contribution should be low, even if the analyzer contributed is only in PoC stage.
2. The level of maturity for each analyzer offered should be clear to end-users. For example, if something cannot cope with large datasets, or is excruciatingly slow, or is not yet thoroughly tested, it should perhaps have a different indicator.

Especially in our kind of community with dynamic participation and commitment, success of (1) necessitates (2).

A CONTRIBUTING.md file can help make (1) happen, and some kind of reviewing/maturity marking indicator should probably be put in place to to achieve (2).