Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Renekton is a very fun champ to play, but his biggest issue is cooldown, and early game squishiness. To counter this, you go offensive quits and reds, and armor yellows, and mr blues.
Your build will go CD - Health/CD - Damage/CD - Damage/Lifesteal - Defense - Defense
Because of your runes and the brutalizer, you have a great amount of armor pen, which allows you to pick on opposing players early and late game. Stacking a Bloodthirster makes sense, because you wont be able to stack much AD with a defensive build, and the lifesteal pays off.
Youre malee, exhaust and Ghost make sense to use with renekton, hes slow, and getting that early exhaust on someone can make them way more gankable early game.