docs(CONTRIBUTING): Add open-source bounty program

Created by: Tirokk

Authored by roschaefer Jan 26, 2020 Merged Jan 29, 2020

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Pullrequest

This should make our open-source bounty program official!

Todo

• Assign amount to each bounty issue with a corresponding label
• Sync our decisions with @DennisHack
• Notifiy our contributors