Add contributing file

[laysauchoa]

What can we help you with?

Hey 🙋🏻‍♀️, do you think is a good idea to add a contributing file with the steps in how to get started contributing to this repository?
I think it makes sense to have some quick HowTo:

• run the tests
• debugging steps
• how to run
• about commit messages
• issues
• others

Where would you expect to find this information?

It should have it's own file and be linked to the readme of the repository

[rominf]

Have you seen: https://github.com/aiven/aiven-client/blob/master/.github/CONTRIBUTING.md?

[laysauchoa]

ah, not really! do you think it make sense to link to the readme?

[rominf]

Yes, I do.

[rominf]

Partial implementation: #261.

[TibsAtWork]

I think there is still a need to expand the CONTRIBUTING file to explain how to setup the development environment in order to allow running the tests.

This needs (at least):

• pip install -r requirements.dev.txt
• pip install requests

(so maybe requests should be added to the requirements.dev.txt file).

Also, the suggested pipenv install --three -e command complains about the -e switch, and I'm assuming we no longer support Python 2? So it might be simpler just to recommend using python -m venv venv to create a virtualenv, and then activate that.

Also explain the expected use of make lint and make reformat (or make validate-style), and the use of make pytest and when new unit tests are expected for a PR.