update readme to reflect upstream project change

The `mr` utilily is now called myrepos upstream and they have a new
website address. Update the links in the readme, reflect the new name
of the project where appropriate, and fix the copy surrounding its
introdiction. The actual command is still `mr`.
update Arch install guide to use stable packages

Encourage the use of the stable packages per disro norms. The devel
packages can be used but they trigger full git clones in many AUR
helpers and are generally a pain if you don't specifically want them for
something. Mention the devel package but update the URLs to point to
the one that uses the normal packaging system.