readme: Update the "packages needed" list
First commit : add missing packages when building on a freshly installed distribution. Fedora:
- Add findutils libtool and libssh-devel
- Replace xxd by vim-common (which does provide xxd)
Debian/Ubuntu :
- Add libssh-dev
(This was tested on latest Docker containers)
Second commit : reformatting. This might be a matter of taste so this commit may be excluded from the merge.