PR - Support token size in has_secure_token

I have created a PR - https://github.com/robertomiranda/has_secure_token/pull/17 in has_secure_token gem, but Robert said “since this is still a back port gem, could you open this PR on https://github.com/rails/rails?”. can anyone suggest me on further steps please. This is my first contribution, so I don’t know how to make PR in rails?? Or I mean, where exactly to make PR in rails?

Thanks