Add a license exception to single owner Nasal libraries which clarifies that merely calling Nasal functions from third party aircraft does not invoke the derived-work clause of the GPL. Signed off by Thorsten

Nasal/orbital_target.nas

@@ -1,9 +1,17 @@
 ###########################################################################
 # simulation of a faraway orbital target (needs handover to spacecraft-specific 
 # code for close range)
+#
+# SPDX-License-Identifier: GPL-2.0-or-later
+#
+# NOTE! This copyright does *not* cover user models that use these Nasal
+# services by normal function calls - this is merely considered normal use
+# of the code, and does *not* fall under the heading of "derived work."
+#
 # Thorsten Renk 2016-2019
 ###########################################################################
 
+
 var orbitalTarget = {
 	new: func(altitude, inclination, node_longitude, anomaly) {
 	        var t = { parents: [orbitalTarget] };