Add in-game controls help

[1shevelov]

at the start of a match.
W,A,S,D and UP, LEFT, DOWN, RIGHT to move,
C to pick up weapon for left player...
Which button is to use a weapon? What else can player do?

[erlend-sh]

V to shoot. That's all the player can do.

[1shevelov]

@erlend-sh and for the second player? I think it's still worth mentioning so that users new that there is nothing else

[dsnopek]

For the second player in "Local" mode, you can use a gamepad, of course, but on the keyboard it's:

• Arrow keys for movement
• L to pickup/throw
• ; to use weapon

When playing online, every player has the first player controls mentioned above (or gamepad).

[dsnopek]

The controls have been added to the README.md per #1.

For a live game, having the controls in-game is a must, but I'm not sure we need to do that for a demo/tutorial. I'll leave this open for now, in any case.

[mojoyup1528]

Im curious how we would actually implement mouse movement for separate cameras or something. The input_buffer is strictly for input actions where mouse events for motion are not available to add as they are not buttons. So when we update the input buffer on the player, could we pass mouse movement separately in the rpc and how would that allow players to not move others with mouse movements?