Add descriptions for all the faps (#13)

.catalog/README.md

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+# Signal Generator
+
+This is a simple signal generator that can be used to generate a signal with a given frequency. There are two modes: PWM and Clock.
+
+## PWM Mode
+
+In PWM mode, the signal is generated by toggling the GPIO pin at the given frequency with a given pulse width. You can also select the GPIO pin on which the signal will be generated.
+
+## Clock Mode
+
+In Clock mode, the signal is generated from the clock signal of the microcontroller. The frequency of the clock signal can be divided by a given value to get the desired frequency. The maximum frequency that can be generated is 64MHz, and the minimum is 32.768kHz. You can also manually select the frequency divider from 1 to 16. The GPIO pin is fixed at 13 (TX).