Update README.md

Hi,

I wondered what you feel about changing the protocol here as I get this error when trying to use git+git rather than the git+http protocol (the former is blocked..)

InstallationError: Command /usr/bin/git clone -q git://github.com/devjones/PyEtherpadLite.git /*/*/*/src/pyetherpadlite failed with error code 128 in None