Did said asshole stop and wait for you? Well then great, get back into the race with them and enjoy a battle with an apologetic driver! Did they not stop and wait? Well then let them know that they knocked you off, but do so after the race. Telling someone that they screwed up isn't going to win you back places, racing will win you back places. Odds are, they didn't notice due to lag and they'll be extremely sorry. Most importantly, don't retaliate no matter how obnoxious someone's driving is, there are better ways to deal with that.
Sometimes, the person who just hit you won't be extremely sorry. Sometimes they'll actually be a real jerk, either through inflammatory comments or through consistent use of ramming, blocking and general dirty racing. The best thing you can do in this case is to get examples of what they said or did and then post them in the appropriate area on the discord. This will not always be the easiest to do since it's hard to know when someone's going to ram you, though programs like Nvidia's Shadowplay may help. If you aren't able to get examples of someone's bad attitude, that's okay! Just try and stay out of that person's way, and eventually they'll end up doing something dumb in front of an admin or someone who is recording and that'll solve that issue.
If you're in a particularly rough and dirty race, you may find yourself getting outright angry at the way people are driving. This is known as going 'on tilt', and it suggests that you're no longer focusing as well on the game because you're instead focusing on how annoying those other drivers are. This is not good for your lap times. You'll find yourself making mistakes more and more, and maybe even causing crashes with other drivers because of it! So, if you get the chance, calm down a bit and think about what you can do to either make up some places or hold onto the one you have right now.