Add installation of Berkshelf in Readme.md

Reading the installation instructions, I miss an important part for the total newbies like me: the installation of Berkshelf (otherwise one has to dig and look for those instructions, which are usually very confusing, since versions 2 and 3 mix up in different pages).

I still couldn't get vagrant-berkshelf working as expected, so perhaps gem install berkshelf -v '3.0.0.rc1' isn't actually what should be done, but if this is wrong then this is a way to humbly ask you how Berkshelf should be installed (thank you in advance!)