Add verification info

[flexxxxer]

Provides a way to verify obtained checking signing cert hash. Signing cert hash does not provided in that PR, HeliBoard developer needs to provide it on their own.

How to obtain signing cert hash? Simple: using keytool command line tool: keytool -list -v -keystore path/to/keystore.jks (you will be asked to provide password)

• you need to know keystore location. also if you used multiple keys you get multiple keys info instead of one.
• you will get output like, and you just need to take what goes after SHA256:

Valid from: <datetime from datetime to>
Certificate fingerprints:
         SHA1: <value>
         SHA256: <another value>
Signature algorithm name: <some signature algorithm name>

keytool usually comes with JDK, in JDK bin folder, see http://stackoverflow.com/questions/5488339/how-can-i-find-and-run-the-keytool

Again, please provide signing key hash on your own using keytool and modifying this PR.

Closes #1852