UART demo, making UART read easier.

gpio/uart_demo/README.md

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+# UART_DEMO
+This is a demonstration of using the UART to read (RX) and transmit (TX) data.  You can connect
+pins RX and TX together with a wire or resistor to create a loopback.  If you have a second 
+Flipper Zero then you can connect TX of the first Flipper Zero to RX of the second and the RX of
+the first to TX of the second.  You also should connect one of the ground pins from the first 
+Flipper Zero to the second Flipper Zero.
+
+When you run the demo application, you will see three options.
+- The first option "Clear" will clear the menu.
+- The second option "Seng Msg 1" will send "Hello World!" follow by new-line delimiter.
+- The third option "Seng Msg 2" will send "Testing 123." follow by new-line delimiter.
+
+Whenever data is received and the delimiter is detected, the new text will be added as an additional
+entry in the menu.
+
+# How it works
+uart_demo.c is the main class that you should look at.  The goal was to simplify all
+the complexity of worker threads and buffers into other files that you should not have
+to look at.
+
+We include uart_helper.h...
+```c
+#include "uart_helper.h"
+```
+
+We create a callback to get invoked whenever a new line (text+delimiter) is detected...
+```c
+// This callback will be invoked whenever a new line is read on the UART!
+static void uart_demo_process_line(FuriString* line, void* context) {
+    // For the demo, we have code to add the line to the submenu.
+}
+```
+
+We configure the UartHelper as follows. We specify the baud rate, the delimiter of our
+message, if we want the delimiter included in our callback data, and which callback to
+invoke.  When data is received by the UART it will be added to a ring buffer.  Once the
+delimiter is detected in the data, our callback will be invoked with the received data.
+The UartHelper uses worker threads and ring buffers to do the processing, but all we need
+to do is call ``uart_helper_alloc`` to start receiving data.
+```c
+#define DEVICE_BAUDRATE 115200
+#define LINE_DELIMITER '\n'
+#define INCLUDE_LINE_DELIMITER false
+
+app->uart_helper = uart_helper_alloc();
+uart_helper_set_baud_rate(app->uart_helper, DEVICE_BAUDRATE);
+uart_helper_set_delimiter(app->uart_helper, LINE_DELIMITER, INCLUDE_LINE_DELIMITER);
+uart_helper_set_callback(app->uart_helper, uart_demo_process_line, app);
+```
+
+We can also send a string using two helper methods...
+```c
+// Send a c-style string
+uart_helper_send(app->uart_helper, "Hello World!\n", 13);
+
+// Send a FuriString
+uart_helper_send_string(app->uart_helper, my_furi_string);
+```
+