If you subscribe to Amazon Prime, you also get a free subscription to Twitch Prime. For the next 3 days, they are giving away Kathy Rain on Twitch Prime!
You do have to install the Twitch Launcher.
To get it, connect your Amazon account to Twitch here:https://twitch.amazon.com/prime
And click where it says "See loot"
On the left of the screen you'll see where you get the game for free, as shown in this screenshot. http://i.imgur.com/QiaCqnWl.png