Created Porting to non-Arduino Platforms (markdown)

Porting-to-non-Arduino-Platforms.md

@@ -0,0 +1,24 @@
+# Porting to non-Arduino Platforms
+
+There are several things that have to be done to port RadioLib to a non-Arduino platform. First, since RadioLib is written in C++, you have to make sure your build system can compile C++ files. Next, you have to make sure your build system DOES NOT define the macro `ARDUINO`. If this macro is defined, RadioLib will attempt to automatically detect which Arduino is being used. If the macro is not defined, RadioLib will attempt to include the file `noarduino.h`, which has to be provided by your build system (e.g. by using GCC's `-I` flag).
+
+The `noarduino.h` file has to define all the platform macros shown in [Custom Build Configuration](https://github.com/jgromes/RadioLib/wiki/Custom-Build-Configuration) - this includes callbacks to Arduino functions, like `digitalRead()`, or `tone()`. Therefore, your platform has to define an equivalent of these functions. For example, in STM32 HAL, there's a function `HAL_GPIO_WritePin()`, which behaves similarly as Arduino `digitalWrite()`, but has an extra argument - the GPIO port. Therefore, an appropriate wrapper should look something like this:
+
+```c++
+// noarduino.h
+// forward declaration
+void wrap_HAL_GPIO_WritePin(uint8_t pin, uint8_t value);
+#define RADIOLIB_CB_ARGS_DIGITAL_WRITE              (void, wrap_HAL_GPIO_WritePin, uint32_t pin, GPIO_PinState value)
+
+// noarduino.c/cpp
+// now the implementation - pin number also includes the GPIOx port
+void wrap_HAL_GPIO_WritePin(uint32_t pin, GPIO_PinState value) {
+  if(pin > 16) {
+    HAL_GPIO_WritePin(GPIOB, uint16_t pin, GPIO_PinState value);
+  } else {
+    HAL_GPIO_WritePin(GPIOA, uint16_t pin, GPIO_PinState value);
+  }
+}
+```
+
+This has to be done for all Arduino functions listed on the page [Custom Build Configuration](https://github.com/jgromes/RadioLib/wiki/Custom-Build-Configuration).