zephyr: Add README.

zephyr/README.md

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+MicroPython port to Zephyr RTOS
+===============================
+
+This is an initial port of MicroPython to Zephyr RTOS
+(http://zephyrproject.org).
+
+The port integrates well with Zephyr build system, using the latest
+features which will be available in 1.6.0, and thus requires Zephyr
+master to build against. All boards supported by Zephyr should be
+supported (but not all were tested).
+
+At this time, only basic interactive prompt (REPL) over UART connection
+is supported. Over time, bindings for various Zephyr subsystems may
+be added.
+
+
+Building
+--------
+
+Follow to Zephyr web site for Getting Started instruction of installing
+Zephyr SDK, getting Zephyr source code, and setting up development
+environment. (Direct link:
+https://www.zephyrproject.org/doc/getting_started/getting_started.html).
+You may want to build Zephyr's own sample applications to make sure your
+setup is correct.
+
+To build MicroPython port, in the port subdirectory (zephyr/), run:
+
+    make BOARD=<board>
+
+If you don't specify BOARD, the default is `qemu_x86` (x86 target running
+in QEMU emulator). Consult Zephyr documentation above for the list of
+supported boards.
+
+
+Running
+-------
+
+To run the resulting application in QEMU (for BOARDs like qemu_x86,
+qemu_cortex_m3):
+
+    make qemu
+
+For deploying/flashing the application on a real board, follow Zephyr
+documentation for a given board.