There's a difference between

    cmd | wc -l

and

    x="$(cmd)"; echo "$x" | wc -l

in that the assignment in the latter removes a trailing newline **if one
exists**, but the "echo" always adds one. This means that if `cmd`
produces no output, the former has 0 lines, and the latter has 1 (empty)
line.

I've not found an easy way to fix this in a way that `wc` can tell the
difference between an empty line and a non-empty one, so I had to find a
separate way to count non-empty lines.

I did try `echo "$x" | grep . | wc -l`, but `shellcheck` complains with
[SC2126](https://github.com/koalaman/shellcheck/wiki/SC2126). The
suggested alternative, `echo "$x" | grep -c .`, causes a failure on
empty inputs (i.e. when there are no upgrades) because `grep` exits with
a failure status if no lines are selected.

My options were then to go with `grep | wc` and silence the warning, or
do `grep -c | cat` to mask the `grep` failure (again). Given that they
both have the same number of commands in the pipe, but `cat` is probably
*slightly* cheaper than `wc`, and the `cat` version doesn't require
adding a `shellcheck`-silencing comment, let's go with that.

You have to explicitly enable it though

Not "randomly" per se (see FixedRandomDelay docs in systemd.timer(7) for
more info) but don't have every user of this application all hammer the
repo exactly on the hour, every hour.

See variants on the "Thundering Herd" problem for more info.

The notification is supposed to read "Reboot when it is convenient - to
install the updates", but I kept reading it as "Reboot - when it is
convenient to install the updates.", and I couldn't figure out the
"correct" punctuation to do that while remaining proper English. So move
the wording about instead.

And document it, along with other installation instructions.

So when you do `make PREFIX=/usr` and then `make install`, it installs
to `/usr`.