More explanation for signature check on README

[sonvirgo]

Thank you for your value source code. It make secret string harder to retrieve for the attacker. But there is still some confuse in signature fingerprint. I think it not like tamper protection right. Unfortunately, attacker send me a clip how he crack and mod my app. I think they tamper the app and your signature check is misleading. I have to now using NDK to read SHA256 signature at runtime to protect against tampering.