Add a reviewer TODO list

[tmigot]

This should help review new problems and not forget about important things

[tmigot]

@arnavk23 Maybe we should add this to the contributing.md ? So that, the person adding new problem can already check if everything is alright ?
Not all points have to be mandatory some might be optional.

[arnavk23]

@tmigot Tangi I think we can close this now. And also #162

[tmigot]

I will do another pass before closing this. I assigned it to me.