cmake readme: fixes to answer the review

Author: 0xc0170

cmake/README.md

@@ -2,22 +2,27 @@
 
 Until further notice, CMake for Mbed OS is still in the development.  
 
-### Targets
+### Supported targets
 
-Only limited set of targets is supported at the moment.
+Only a limited set of targets is supported at the moment.
 
 These targets should work:
 - K64F 
 - DISCO_L475VG_IOT01A 
 - NRF52840_DK 
 - WIO_3G
+- K66F
 
-We will be adding more targets.
+### Supported toolchains
+
+ARMClang6 and Gcc Arm toolchains are supported.
 
 ### Examples
 
-Supported examples must contain CMakelists.txt. If it is not present, an example needs to be ported to support CMake.
+Supported examples can be identified by the presence of a top level `CMakelists.txt` input source file. 
 
-### Contributions are welcome!
+### Known issues
 
+- ARMClang6: the assembler does not include definitions. It will be fixed in the upcoming release (>3.18.2)
+- Ninja: ARMClang6 response files require unix paths on Windows, see https://gitlab.kitware.com/cmake/cmake/-/issues/21093