Add signature hash for verification via appverifier

[twitchyisreal]

Since the main installation method is obtanium appverifier is essential so a hash of the signature should be released in the README and on another website

[jegly]

Both branches are signed with the same key.

Certificate SHA-256: 8346b1a70d09ff5c9f7d7febc874cf694b6e267032a4eb38e261d538bce7b09c

Verify with apksigner:
apksigner verify --print-certs Box_*.apk | grep SHA-256

in future i can pin to HF also, Great suggestion! thank you :)