Add "hack/install.sh" to the "PRs welcome" note in the end of the install script

Also, simplified the output method by using `cat >&2 <<EOF ...` instead of multiple lines of `echo >&2 '...'`.

Signed-off-by: Andrew Page <admwiggin@gmail.com>
Merge pull request #8085 from tianon/hack-install-location

Add "hack/install.sh" to the "PRs welcome" note in the end of the install script