add boilerplate to all testcases with documentation references

testcases/t/174-regress-focus-toggle.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)
+#
 # Regression: Checks if i3 still lives after using 'focus mode_toggle' on an
 # empty workspace. This regression was fixed in
 # 0848844f2d41055f6ffc69af1149d7a873460976.