esp8266: Add README.md.

esp8266/README.md

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+Micro Python port to ESP8266
+============================
+
+This is a port of Micro Python to the Espressif ESP8266 wifi module.
+
+Currently implemented features include:
+- REPL (Python prompt) over UART0.
+- 24k heap RAM available for Python code.
+- Garbage collector, exceptions.
+- Unicode support.
+- Builtin modules: gc, array, collections, io, struct, sys.
+- C long-long type used as bignum implementation (gives 64 signed ints).
+
+Note that floating-point numbers are not supported.
+
+On the TODO list:
+- Wifi support.
+- GPIO support.
+- Internal filesystem using the flash.
+- ...
+
+Build instructions
+------------------
+
+The tool chain required for the build is the OpenSource ESP SDK, which can be
+found at <https://github.com/pfalcon/esp-open-sdk>.  Clone this repository and
+run `make` in it's directory to build and install the SDK locally.  T
+
+Then, to build Micro Python for the ESP8266, just run:
+```bash
+$ make
+```
+This should produce binary images in the `build/` subdirectory.  To flash them
+to your ESP8266, use:
+```bash
+$ make deploy
+```
+This will use the `esptool.py` script to download the images.  You must have
+your ESP module in the bootloader, and connected to a serial port on your PC.
+The default serial port is `/dev/ttyACM0`.  To specify another, use, eg:
+```bash
+$ make PORT=/dev/ttyUSB0 deploy
+```
+
+The images that are built are:
+- `firmware.elf-0x00000.bin`: to be flashed at 0x00000
+- `firmware.elf-0x10000.bin`: to be flashed at 0x10000
+
+There is also a combined image, made up of the above 2 binary files with the
+appropriate padding:
+- `firmware-combined.bin`: to be flashed at 0x00000