README: Document that librpm is required for Fedora etc.

This fixes commit 5e85a311a866e691715fe4450b53f5655e4d5ae4.
rpm: Fedora: Allow use of 'dnf download' if 'yumdownloader' is not available (RHBZ#1156498).

Prefer yumdownload because dnf download doesn't work reliably
in parallel (RHBZ#1157233).