This adds the packages that provide aria2c to the prerequisite lists for
Linux distributions.
It also gives instructions for MacPorts on macOS. MacPorts is listed
before Homebrew because it should be considered preferred: all packages
needed here are available in the standard MacPorts repository, as
opposed to Homebrew which requires the use of an external source to
install an outdated version of chntpw.
* This resolves issue #38
* This ensures that the aria2 homebrew package is installed, which
provides the `aria2c` command used to download files.
* This only changes instructions in the readme file, there are no actual
code changes in this commit/PR.
