Add README.md

Author: evgeniy-paltsev

README.md

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+# What this is not
+This repository isn't downstream ARC Zephyr fork, all improvements and fixes for ARC cores are submitted to upstream project.
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+Please visit https://github.com/zephyrproject-rtos/zephyr/releases if you're looking for Zephyr releases or https://github.com/zephyrproject-rtos/zephyr if you're interested in a bleeding edge sources.
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+This repository isn't the place to submit issues or pull-requests (even ARC-related ones). If you want to submit issue or pull-request please visit https://github.com/zephyrproject-rtos/zephyr
+
+# What you may find here still
+You can find here some work-in-progress development branches which haven't reach the upstream yet.