add boilerplate to all testcases with documentation references

testcases/t/118-openkill.t

@@ -1,6 +1,19 @@
 #!perl
 # vim:ts=4:sw=4:expandtab
 #
+# Please read the following documents before working on tests:
+# • http://build.i3wm.org/docs/testsuite.html
+#   (or docs/testsuite)
+#
+# • http://build.i3wm.org/docs/lib-i3test.html
+#   (alternatively: perldoc ./testcases/lib/i3test.pm)
+#
+# • http://build.i3wm.org/docs/ipc.html
+#   (or docs/ipc)
+#
+# • http://onyxneon.com/books/modern_perl/modern_perl_a4.pdf
+#   (unless you are already familiar with Perl)
+#
 # Tests whether opening an empty container and killing it again works
 #
 use List::Util qw(first);