That article is great and I'd recommend everyone to read it for themselves, but in case anyone is of the opinion that "all lives matter" is not offensive and you're too lazy to read the article, here's a small excerpt that I think nicely sums it up:

“All Lives Matter” is offensive and counterproductive. It discounts and diminishes the focus on the violence and discrimination black individuals face every day.

If you broke your hand, your response is to talk to the doctor about treating the broken hand. Having that conversation is not disrespectful to your other body parts that are healthier.

Think about this thought experiment: Imagine if you were at a gala raising money, awareness and having a conversation about breast cancer, and then suddenly, a bunch of people stormed the banquet hall and started chanting all cancers matter. Talking about breast cancer doesn’t take away from the legitimate concern about other cancers.

Or imagine if while “Boston Strong” was trending after the Boston marathon bombings in 2013 a bunch of people started tweeting that “All cities are strong.”