Add usage section to README for new interactions. Closes #11.

Author: ezod

README.md

@@ -7,6 +7,19 @@ A simple Flipper Zero application for NMEA 0183 serial GPS modules, such as the
 
 Heavy lifting (NMEA parsing) provided by [minmea].
 
+## Usage
+
+This is a single-screen app, and a few interactions are provided via the
+hardware buttons:
+
+- Long press the up button to change the **baud rate**. The default baud rate
+  is 9600, but 19200, 38400, 57600, and 115200 baud are also supported.
+- Long press the right button to change **speed units** from knots to
+  kilometers per hour.
+- Press the OK button to set the **backlight** to always on mode. Press it
+  again to disable.
+- Long press the back button to **exit** the app.
+
 ## Hardware Setup
 
 Connect the GPS module to power and the USART using GPIO pins 9 (3.3V), 11