added a comment in features/pom.xml

added a comment in features/pom.xml

This comment avoids being tempted to fix a compilation warning with
a kar packaging.According to the developper guide, the inherited
jar packaging must be used instead. But it does not remove the warning.

https://bugs.opendaylight.org/show_bug.cgi?id=2760
http://docs.opendaylight.org/en/stable-boron/developer-guide/odl-parent-developer-guide.html

Change-Id: I35438fd73530b9c7abe08dcfb12a73a4c3ae6925
Signed-off-by: guillaume.lambert <guillaume.lambert@orange.com>