Add contributing.md

[drillprop]

As the title says, it would be nice to list steps how to contribute to this project.

There are some templates on github like, for example, this one. I'm not sure what will fit to this project and how detailed should be. Maybe someone has bigger experience in contributing to open source. I think it requires discussion. I'll try to send some inspirations/templates too.

[drillprop]

Something like this one looks very nice. Simple and straightforward.

[typeofweb]

I'm on it!