readme update

Author: robamu

README.md

@@ -7,6 +7,17 @@ This repostiory contains the first version of a possible RTEMS CMake build auppo
 Include the `RTEMSGeneric.cmake` file in CMake and call the `rtems_generic_config` function, passing the RTEMS installation path (also commonly called prefix)
 and the RTEMS BSP (e.g. sparc/erc32) to this function.
 
+## Extending the build system support
+
+This is still a prototype. It has been tested for the Hello World demo from the RTEMS quick start guide which uses the `sparc/erc32` BSP and for a STM32 blinky using the `arm/stm32h7` BSP.
+
+Extending the build support is relatively easy: 
+
+Extract the necessary compiler and linker flags for the RTEMS build from the pkgconfig file
+for the specific BSP. This file will generally be located inside the `lib/pkgconfig` folder of the RTEMS tools folder. Add these flags in the `RTEMSHardware.cmake` file for your specific BSP.
+
+When building with CMake, `-v` can be added to verify the flags.
+
 ## Example
 
 See https://github.com/rmspacefish/rtems-demo/tree/master/applications/hello for an example. This is the Hello World project taken from the RTEMS quick start guide,