Update README.md

better readme

Author: robamu

README.md

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 # rtems-cmake
 
 This repostiory contains the first version of a possible RTEMS CMake build aupport. The intention is to export the RTEMS specific configuration of the cross compiler toolchain and the flags to external `*.cmake` files so the application `CMakeLists.txt` can stay clean, similarly to the `rtems_waf` support.
+
+## How to use
+
+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.