Update README.md

Author: fpistm

README.md

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-# arm-none-eabi-gcc
-The GNU Arm Embedded Toolchain binaries used by STM32duino core supported by ST
+# The GNU Arm Embedded Toolchain binaries used by the [STM32 core](https://github.com/stm32duino/Arduino_Core_STM32) support for Arduino
+
+## Rationale
+
+Provide GNU Arm Embedded Toolchain binaries required during the installation of the [STM32 core](https://github.com/stm32duino/Arduino_Core_STM32) support for Arduino which required 4 platforms.
+
+Toolchain is automatically installed when the STM32 core is installed thanks the [Arduino Boards Manager](https://www.arduino.cc/en/guide/cores) --> See [stm32 core installation](https://github.com/stm32duino/wiki/wiki/Getting-Started#installing-stm32-cores) wiki page.
+
+This is **not** a new GCC toolchain distribution for ARM devices. 
+Official [GNU Arm Embedded Toolchain](https://developer.arm.com/open-source/gnu-toolchain/gnu-rm) distribution, by ARM.
+
+## Changes
+
+Compared to the original ARM release, there are no changes. The **same architecture options** are supported and **the same 
+combinations of libraries** (derived from newlib) are provided.
+
+
+## Releases
+
+A release is done when an update of the **GNU Arm Embedded Toolchain** is required for the [STM32 core](https://github.com/stm32duino/Arduino_Core_STM32) support for Arduino.
+It generally use the latest official [GNU Arm Embedded Toolchain](https://developer.arm.com/open-source/gnu-toolchain/gnu-rm) 
+releases, maintained by ARM.
+
+### 6-2017-q1-update
+
+This release is equivalent to **6-2017-q1-update** from February 23, 2017,
+and is based on `gcc-arm-none-eabi-6-2017-q2-update-src.tar.bz2`.