README: added details to generate self-signed cert

Hello,

Thank you for developing this nice privacy and compression proxy :-)

I'm just proposing here to improve the README file and add details about how to generate a self-signed certificate. Every time I have to generate a new key and certificate, I have to find the correct options :-)

I also added a link to the next section: how to validate the self-signed certificate on Firefox? I didn't see these details maybe because I already used Janus Configurator Proxy add-on before and I thought that there was something wrong with my certificate... It was my fault but I hope these new details will help other absent-minded people :-)

Last details: I added the minimal version for Firefox.

If you want, I just created a Dockerfile and a docker repository to quickly build `node-janus` on a container:

* https://github.com/matttbe/docker-janus-node
* https://registry.hub.docker.com/u/matttbe/docker-janus-node/

Have a nice day,

Matt