Added playstore link, SHA1 & SHA256 signing certificate fingerprints to README

[DanyPM]

Add the app certificate fingerprint to the README file, enabling apps and users to check the integrity of apk installed from github.

The main purpose is to enable subsequent verification of the app through AppVerifier, including adding the fingerprint to its internal database.

Especially relevant given:

• the probable future integration in F-droid #2
• the distribution of releases on the repository #3 #6