CMake: Add background and future plans to README (#13589)

cmake/README.md

@@ -1,20 +1,31 @@
 ## CMake for Mbed OS
 
-Until further notice, CMake for Mbed OS is still in development. 
+In order to [replace the traditional Mbed tools](https://os.mbed.com/blog/entry/Introducing-the-new-Mbed-Tools/), we are adding CMake support to allow the generation of build systems instructions to build Mbed OS as a library.
+
+This is still at the experimental stage and still in development. It does not yet completely replace our current build tools. If you find a bug, please report it or raise a pull request as contributions are welcome!
+
+Mbed OS is built as collection of all libraries supported by a given Mbed target. Application project executables can be linked with the monolithic `mbed-os` library to build Mbed applications.
+
+We are currently working to break down the `mbed-os` library into several individual libraries that applications can selectively choose to include depending on their needs.
+
+The following features are not yet supported and will be progressively added:
+* Application profile selection (`bare metal`)
+* C library selection (`small`)
+* `s/f/v/printf` library selection (`minimal-printf`)
+* Other features that require altering compiler/linker command line options
 
-Application profile selection (`bare metal` or `full`), printf library selection (`minimal-printf` or `std`), C library selection (`small` or `std`) and other features that require altering compiler/linker command line options are not yet supported.
 The full profile with the standard printf and C library are selected by default.
 
 ### Supported targets
 
 Only a limited set of targets is supported at the moment.
 
-These targets should work:
-- K64F 
+The following targets are supported:
 - DISCO_L475VG_IOT01A 
+- K64F 
+- K66F
 - NRF52840_DK 
 - WIO_3G
-- K66F
 
 ### Supported toolchains