ACK, No Milk! Cats are lactos intolerant, milk will make it sick.
The best thing to do would call a local sheltar adn have them come get the kitten.
Also ask your neighbours where the cat was found if anyone is missing a kitten, there could be a family right now devistated that they cannot find there kitty.
I had a dog that used to jump the fence (actually i have another dog that does the same thing argh) and he was never a "stray" but some lady just asumed that once and tryed keeping our dog! So you never know.
Best to call the SPCA or local sheltar, it's not fare to leave a young cat outside with no warmth or food.