Created Custom Build Configuration (markdown)

Custom-Build-Configuration.md

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+# Custom Build Configuration
+
+RadioLib supports many different Arduino cores, but sometimes it may be beneficial to define your own "platform". This can be achieved by frist defining the macro `RADIOLIB_CUSTOM_PLATFORM` and then defining all the required platform specification parameters. All the definitions **MUST** be placed in the Arduino sketch prior to including the main library file RadioLib.h, as is shown in the following example.
+
+```c++
+#define RADIOLIB_CUSTOM_PLATFORM
+
+// the following parameters must always be defined
+#define RADIOLIB_PIN_TYPE                           uint8_t
+#define RADIOLIB_PIN_MODE                           uint8_t
+#define RADIOLIB_PIN_STATUS                         uint8_t
+#define RADIOLIB_INTERRUPT_STATUS                   RADIOLIB_PIN_STATUS
+#define RADIOLIB_DIGITAL_PIN_TO_INTERRUPT(p)        digitalPinToInterrupt(p)
+#define RADIOLIB_NC                                 (0xFF)
+#define RADIOLIB_DEFAULT_SPI                        SPI
+
+// the following parameters must be defined if the Arduino core does not support SoftwareSerial library
+#define RADIOLIB_SOFTWARE_SERIAL_UNSUPPORTED
+#define RADIOLIB_HARDWARE_SERIAL_PORT               Serial1
+
+// the following parameters must be defined if the Arduino core does not support tone function
+#define RADIOLIB_TONE_UNSUPPORTED
+
+#include <RadioLib.h>
+```