package/Config.in: explain that lua package names should start with lua

package/Config.in: explain that lua package names should start with lua

We have decided that lua packages should have a name that starts with
lua (like is the case for python and perl). However, we're not going to
rename all the existing lua packages that don't start with lua. This
makes it unclear for people adding packages how they should name the
package, so add a comment to package/Config.in to explain it.

It's rather terse but it gets the message across.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>