README: added details to generate self-signed cert

The goal is to show that the user has to regenerate a new key and a
new certificate.

It can be interesting to also add a link in this new section to generate
a self-signed certificate to the instructions about how to validate this
new certificate in Firefox. I guess that like me most users already have
installed and configure Janus Configurator Proxy Add-on and they will
not read the section to set the new PAC URL.
README: Firefox: minimal version = 33

And 33 is the current Beta and not the current Nightly version