add CONTRIBUTING.md
add CONTRIBUTING.md

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

related to #81, #68, #31

koebi/i3lock,ravinrabbid/i3lock-svg,kuruczgyurci/i3lock,kuruczgyurci/i3lock,sebastian-frysztak/i3lock-color,sebastian-frysztak/i3lock-color,ravinrabbid/i3lock-svg,kuruczgyurci/i3lock