Created KISS Extended API (markdown)

KISS-Extended-API.md

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+KISS command extensions are used for radio module control and signal report events on port 0, command for radio control is defined as 6 (KISS SetHardware) and signal report command as 7. Radio modules/modems can implement these commands, so they will be controllable from the application and application will be able to show signal levels on S-meter.
+
+Payloads for commands are sent and expected as big endian and defined as:
+```
+  // KISS SetHardware 6
+  struct SetHardware {
+    uint32_t freq;
+    uint32_t bw;
+    uint16_t sf;
+    uint16_t cr;
+    uint16_t pwr;
+    uint16_t sync;
+    uint8_t crc;
+  } __attribute__((packed));
+  
+  // KISS command 7
+  struct SignalReport {
+    int16_t rssi;
+    int16_t snr;  // snr * 100
+  } __attribute__((packed));
+
+  // KISS command 8
+  struct RebootModem {
+  } __attribute__((packed));
+`
+```