contribution.md: write contribution rules

[aristov]

We should explain some thesises like:

• We try to avoid to solve "because we can" problems, until a real world problem occurred
• Until there are no real cases which make block almost unusable, no API changes are welcome

formulated by @narqo in #1126

Also I propose to add a second point: "Assign the issue to yourself".

[aristov]

cc @veged, @dfilatov, @narqo, @tadatuta What else should we add to the list above?

[aristov]

Also I propose to add a second point: "Assign the issue to yourself".

But I have no idea, how to assign an issue if contributor is not a member of BEM team, e.g. I can not assign https://github.com/bem/bem-core/issues/594 to @apsavin, who made PR for it. And if I right understand he can't do it by himself.

[narqo]

Also I propose to add a second point: "Assign the issue to yourself".

I think, that assignation should be interpreted as a responsibility for the end result. You can't give such responsibility to people outside of the team, as well as you can't demand anything from the contributors. In other cases no one become responsive.

What else should we add to the list above?

Maybe @mishanga with @tadatuta could have a meeting with you and help you to formalise the draft?

[qfox]

I think we also need some maintainers guide. Just for consistency.