cmake readme: add how to build

Author: 0xc0170

cmake/README.md

@@ -26,3 +26,13 @@ Supported examples can be identified by the presence of a top level `CMakelists.
 - ARMClang6: the assembler does not include definitions. It will be fixed in the upcoming release (>3.18.2). Reference: https://gitlab.kitware.com/cmake/cmake/-/issues/21148#note_823305
 - Ninja: ARMClang6 response files require unix paths on Windows. Reference: https://gitlab.kitware.com/cmake/cmake/-/issues/21093
 
+## How to build an application
+
+Prerequisities:
+- CMake >3.18.2
+- mbed-tools >3.2.0
+
+From the application root run the following:
+1. `mbedtools configure -m target_name -t toolchain name` - this creates Mbed OS configuration
+2. `mkdir build && cd build && cmake .. -GNinja && cmake --build .`
+