Add contributing guide and CODEOWNERS

[chshersh]

We can take example from importify:

• https://github.com/serokell/importify/blob/master/CONTRIBUTING.md

And add extra things, like, don't forget to modify .hlint.yaml file. Maybe something else...

And we also should add CODEOWNERS file:

• https://github.com/serokell/importify/blob/master/CODEOWNERS

With the following CODEOWNERS:

• @ChShersh is a codeowner of everything
• @vrom911 is a codeowner of .hlint.yaml and benchmarks
• @johnchandlerburnham is a codeowner of test/Test/Universum + test/Spec.hs and shell.nix