add Signing certificate fingerprint to readme

[mahibi]

• resolve List Signing Certificate Fingerprint to Let Users Verify the Downloaded APK #5485

🏁 Checklist

• ⛑️ Tests (unit and/or integration) are included or not needed
• 🔖 Capability is checked or not needed
• 🔙 Backport requests are created or not needed: /backport to stable-xx.x
• 📅 Milestone is set
• 🌸 PR title is meaningful (if it should be in the changelog: is it meaningful to users?)

[github-actions]

APK file: https://www.kaminsky.me/nc-dev/android-artifacts/5489.apk



To test this change/fix you can simply download above APK file and install and test it in parallel to your existing Nextcloud app.

[github-actions]

Codacy

Lint

Type	master	PR
Warnings	99	98
Errors	0	0

SpotBugs

Category	Base	New
Bad practice	6	6
Correctness	11	11
Dodgy code	62	62
Internationalization	3	3
Malicious code vulnerability	3	3
Performance	4	4
Security	1	1
Total	90	90