You can find this small tutorial text in the game as well, by pressing F1.
Basics
It's all about 
versus 
A small group of Traitors is randomly picked.
Together they have to kill all the Innocent.
The Innocent do not know who is a Traitor and who isn't.
The Traitors must use this fact, because they are outnumbered.
Death
If you die, you will not respawn until next round.
Innocents will not know you are dead until they find your corpse.
Corpses may have information that leads to the killer.
Special Equipment
Traitors can buy special equipment.
Their Equipment Menu can be opened by pressing C.
Innocent players who were selected as Detectives may also buy
special detective equipment, which can help them find the Traitors.
Tips
As Innocent, do not kill too easily! Killing other Innocents costs you a lot of points and Karma.
Make sure you're shooting a Traitor.
As Traitor, hide the body! Innocents get valuable info from corpses.
As Traitor, communicate and cooperate! Cause distractions,
lead targets into traps, cover up kills made by other traitors.
As Detective, search bodies! All Innocents can
see and use the results you find.
Keys
Q will drop your weapon so you can pick up another.
C will open the Equipment Menu, if you are Traitor or Detective.
Shift sends voicechat only to other Traitors.
F1 shows the Help and Settings menu.
Set a key for Suit Zoom in your settings to use Radio Commands.
Karma
Your Karma starts at 1000 and goes down if you damage players who are on your own side.
It goes down less if their Karma is lower.
If your Karma is low when a round starts, you get a penalty to your damage that round.
By playing clean rounds where you don't harm teammates, you regain Karma.
Innocent also get some for hurting Traitors.
The Karma value shown on the scoreboard updates only after the round ends.
During a round, someone's true Karma might be lower.
Check the full Help for more details.
