Edit installation from source docs

Created by: voisey

When I followed these instructions I found that one no longer needs to install the C++ compiler themselves, in particular the compiler is installed automatically when doing conda install --only-deps nnpdf on both linux and mac, so step 2 is no longer necessary. I also checked whether any of the packages mentioned in what was step 4 are already installed and pkg-config is, so I got rid of any mention of this.