Add CONTRIBUTING.md

Author: ThomasGravekamp

CONTRIBUTING.md

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+# Contributing to STM32-base-STM32Cube
+
+No active development is happening in this repository. This repository is just a mirror of the CMSIS and HAL files for STM32 microcontrollers. These files have been copied from the [STM32Cube packages](https://www.st.com/en/embedded-software/stm32cube-mcu-packages.html) as published by [STMicroelectronics](https://www.st.com) and the files have been left untouched.
+
+This means that any pull requests on this repository will be ignored. However, if you notice that there is a newer version of one of the STM32Cube package available, please send a message so that these packages can be updated as soon as possible.