Created Modem control using KISS protocol extension (markdown)

Modem-control-using-KISS-protocol-extension.md

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+When `EnableKissExtensions` configuration parameter is set to `true` modem will be able to handle SetHardware KISS command 6 for setting modem parameters and will send event about signal level with KISS command 7, both are operating on KISS port 0. This way client application (such as [Codec2 Walkie-Talkie](https://github.com/sh123/codec2_talkie)) can display signal levels and change modem parameters dynamically.
+
+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 Reboot {
+  } __attribute__((packed));
+```