Add F-droid Link to ReadMe

[skyGtm]

Description

BEFORE: "Get it on F-droid" button just shows the thumbnail in full screen. AFTER: "Get it on F-droid" button takes to correct F-droid page of Clima.

### References

Testing

• [ ] This change adds test coverage for new/changed/fixed functionality

Checklist

• [x] The correct base branch is being used, if not master