Document fixed time-step

Docs/configuring_the_simulation.md

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+<h1>Configuring the simulation</h1>
+
+Before you start running your own experiments there are few details to take into
+account at the time of configuring your simulation. In this document we cover
+the most important ones.
+
+For the full list of settings please see [CARLA Settings](carla_settings.md).
+
+Fixed time-step
+---------------
+
+The time-step is the _simulation-time_ elapsed between two steps of the
+simulation. In video-games, this _simulation-time_ is almost always adjusted to
+real time for better realism. This is achieved having a **variable time-step**
+that adjusts the simulation to keep up with real-time. In simulations however,
+it is better to detach the _simulation-time_ from real-time, and let the
+simulation run as fast as possible using a **fixed time-step**. Doing so, we are
+not only able to simulate longer periods in less time, but also gain
+repeatability by reducing the float-point arithmetic errors that a variable
+time-step introduces.
+
+CARLA can be run in both modes.
+
+<h4>Variable time-step</h4>
+
+The simulation tries to keep up with real-time. To do so, the time-step is
+slightly adjusted each update. Simulations are not repeatable. By default, the
+simulator starts in this mode
+
+<h4>Fixed time-step</h4>
+
+The simulation runs as fast as possible, simulating the same time increment on
+each step. To run the simulator this way you need to pass two parameters in the
+command-line, one to enable the fixed time-step mode, and the second to specify
+the FPS of the simulation (i.e. the inverse of the time step). For instance, to
+run the simulation at a fixed time-step of 0.2 seconds we execute
+
+    $ ./CarlaUE4.sh -benchmark -fps=5
+
+It is important to note that this mode can only be enabled when launching the
+simulator since this is actually a feature of Unreal Engine.
+
+Synchronous vs Asynchronous mode
+--------------------------------
+
+> TODO