README.md: Clarify when dh-exec is NOT useful in an install file

In case the .install script uses only ${DEB_HOST_MULTIARCH}, and has a
single component, the native wildcard support of debhelper will most
likely suffice, and there's no need for dh-exec. Make this clear in
the README.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>