Update docs to include instructions for the algorithm parameter.
Update docs to include instructions for the algorithm parameter.

This parameter is necessary to prevent attackers from bypassing the
algorithm verification step.

See:
https://auth0.com/blog/2015/03/31/critical-vulnerabilities-in-json-web-token-libraries/
https://github.com/jwt/ruby-jwt/issues/76
https://github.com/jwt/ruby-jwt/pull/71
https://github.com/jwt/ruby-jwt/issues/107

excpt/ruby-jwt,progrium/ruby-jwt,yahooguntu/ruby-jwt,ab320012/ruby-jwt,jwt/ruby-jwt