All words were considered "slang" at once point in time, words don't just pop out of the ground. So, if it's a word a lot of people use, why shouldn't it be in the dictionary? After all, the English language changes all the time, it should make room for modern words.
Actually, "selfie" already is in the dictionary:
Any word that is used by people should be in the dictionary. Why not? Dictionaries allow people old and young to learn what is meant by a word. Slang and informal terms may be even less understood universally than others and thus should be explained.