Add information and links to using ARMC6

Author: bridadan

docs/tools/tools_intro.md

@@ -21,7 +21,10 @@ We created the Mbed command-line tool (Mbed CLI), a Python-based tool, specifica
 
 Mbed OS 5 can be built with various toolchains. The currently supported versions are:
 
-- [Arm Compiler 6.11 (default ARM toolchain)](https://developer.arm.com/products/software-development-tools/compilers/arm-compiler/downloads/version-6), also included in [Keil MDK 5.27](http://www2.keil.com/mdk5/).
+- Arm Compiler 6.11 (default ARM toolchain).
+  - A free version when it's used with Mbed OS is included with [Mbed Studio](https://os.mbed.com/studio/). For information on using this with Mbed CLI, please see the [After installation - configuring Mbed CLI page](../tools/after-installation-configuring-mbed-cli.html).
+  - A paid version is available as [Arm Compiler 6.11 Professional](https://developer.arm.com/products/software-development-tools/compilers/arm-compiler/downloads/version-6).
+  - A paid version is also included in [Keil MDK 5.27](http://www2.keil.com/mdk5/).
 - [Arm Compiler 5.06 update 6 (to be deprecated in the future)](https://developer.arm.com/products/software-development-tools/compilers/arm-compiler-5/downloads).
 - [GNU Arm Embedded version  6 (6-2017-q1-update)](https://developer.arm.com/open-source/gnu-toolchain/gnu-rm/downloads).
 - [IAR Embedded Workbench 8.32.1](https://www.iar.com/iar-embedded-workbench/tools-for-arm/arm-cortex-m-edition/).