Add instructions to fully working diffoscope from git clone / tarball

Currently only in pip installation method there is comment that mention installing missing tools:

Note: You might still want to install Python modules from tlsh and rpm and other external tools to get more meaningful results. Use diffoscope --list-tools to get the full list.

but not after/for Source tarballs or Through Git options, see related issue: (when fixed the instructions can be changed to use the new script/options) diffoscope#161