Add instructions for image-based tests to the contributors guide

[benmccann]

This is mostly borrowed from #3988 along with incorporating some info from recent questions asked in the Slack channel

[nagix]

This is very helpful for new contributors!

[kurkle]

Couple of things that sound wrong to me. I also noticed that debug is not needed for new tests, because its going to fail anyway when the png is missing. Only really need debug when trying to figure out why some test doesn't fail, right?

[benmccann]

You're right. I just tested and debug doesn't seem necessary. I removed that step from the instructions

[Vincent-Ip]

Hi @benmccann, I was using this updated guide to add an image test for a new feature. This guide was pretty easy to follow. I did have just some minor comments to add to this PR. Hopefully they are helpful.

[kurkle]

I thin the debug option should still be documented.

[benmccann]

Ok. Added a line about the bug option

[simonbrunel]

Thanks @benmccann, I should have wrote these guidelines long time ago :)