Add a (hopefully) clarifying comment

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
ij-legacy: prevent ImageJ 1.x from parsing -sources.jar files

The -sources.jar files are most likely generated by Maven (e.g. when
imitating Fiji plugins) and they might contain plugins.config files
that ImageJ 1.x should *not* misinterpret as meaning that this .jar
contains any plugins at all.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Merge pull request #11 from imagej/legacy-sources-jars

Force ImageJ 1.x to ignore -sources.jar files