Unclear reference to JwtStore class in readme docs

[mathiasconradt]

In the readme docs on https://github.com/auth0/java-jwt a class JwtStore is used in the sample code, but it's unclear where (which package) this class is coming from. Some clarification on how to construct an instance of this class would be helpful, since it does not seem to be part of the java-jwt project.

final JwkStore jwkStore = new JwkStore("{JWKS_FILE_HOST}");