Add info for contributors

[Petr Messner]

What info should be provided:

• what is this project
• how can anybody help
• how to ask
• who is responsible/owner/primary contact for what
• ...? :)

Some links:

• https://help.github.com/en/articles/setting-guidelines-for-repository-contributors
• https://help.github.com/en/articles/about-code-owners

Ideas:

• examine cpython github repo for their github infrastructure
• https://pyvec.slack.com/archives/C7FAR7HBL/p1551039126014700