README: Firefox: minimal version = 33

And 33 is the current Beta and not the current Nightly version
Conflicts:
	README.md
Merge pull request #61 from MPTCP-smartphone-thesis/readme-firefox

README: Firefox: minimal version = 33
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 when using `node-janus` in an external server.