Pin 5 is control--you can control the operation of the timer from an outside source. The Control is often used for audio modulation, but for a simple square wave generator it is usually not used. However, if you leave the pin unconnected, it can pick up electromagnetic waves that can cause a tiny bit of erratic behavior, which is often undesired. If you're only using the chip to flash an LED, for example, you probably don't need the capacitor. You probably won't see the difference. The .01uF cap is only there to eliminate the unwanted signals.
Der Strom