contribution.md: write contribution rules

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".

Open contribution.md and review the existing contribution guidance. Add the stated principles about avoiding premature solutions and API changes, along with the proposal to assign an issue to yourself. Done means all three rules are clearly and consistently included in the contribution documentation.

Written by the indexing model from the issue text.