Add CONTRIBUTING.md

[sobolevn]

For future contributors CONTRIBUTING.md might be very useful.
Here are some examples:

• https://github.com/wemake-services/wemake-python-styleguide/blob/master/CONTRIBUTING.md
• https://github.com/mimesis-lab/pytest-mimesis/blob/master/CONTRIBUTING.md

[sobolevn]

@kaelscion please, share your experience with other contributors. Does these examples seem complete to you?

[kaelscion]

The python-styleguide CONTRIBUTING.md makes the most sense for the dotenv-linter. Do you mind if I use that one as a template for the CONTRIBUTING.md here?

[sobolevn]

Sure, feel free.

[kaelscion]

Sorry for the large amount of commits for the same file. I kept making small markdown errors and needed to fix them. All good now though!

[sobolevn]

Now we can start working on the main scope of the project.

See #1