Update README to make it clear it's a fork from Planka

[bzg]

• What are the reasons for the fork?
  • Did you have interactions with the upstream Planka team
• What is the latest commit from upstream Planka repo?
• What are the purpose for this fork?
  • Do you plan to accept contributions?
  • Do you plan to work on new features?
• Are there other forks and if so, what are their purpose?