This change adds uxntal.7 [1].
It also updates the makefile to install the executables and man page
in ~/.local (~/.local/bin and ~/.local/share/man respectively). This
can easily be overridden by using PREFIX, for example:
# install into home directory
make PREFIX=$HOME install
# install into /usr/local
make PREFIX=/usr/local install
Finally, it updates the README to describe these things.
[1] Origin: https://git.phial.org/d6/nxu/raw/branch/main/uxntal.7
