add CONTRIBUTING.md

…so that people know we don’t want to extend i3lock’s image handling.

related to #81, #68, #31

.github/CONTRIBUTING.md

@@ -0,0 +1,3 @@
+Please do not send/request features related to image manipulation. i3lock’s
+support for background images is intentionally kept minimal, you should do all
+pre-processing in external tools.