ESP8266AVRISP: add README.md

Author: kzyapkov

libraries/ESP8266AVRISP/README.md

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+# AVR In-System Programming over WiFi for ESP8266
+
+This library allows an ESP8266 module with the HSPI port available to become
+an AVR In-System Programmer.
+
+## Hardware
+
+The ESP8266 module connects to the AVR target chip via the standard 6-pin
+AVR "Recommended In-System Programming Interface Connector Layout" as seen
+in [AVR910](http://www.atmel.com/images/doc0943.pdf) among other places.
+
+If the AVR target is powered by a different Vcc than what powers your ESP8266
+chip, you **must provide voltage level shifting** or some other form of buffers.
+Exposing the pins of ESP8266 to anything larger than 3.6V will damage it.
+
+Connections are as follows:
+
+ESP8266 | AVR / SPI  
+--------|------------
+GPIO12  | MISO
+GPIO13  | MOSI
+GPIO14  | SCK
+any*    | RESET
+
+For RESET use a GPIO other than 0, 2 and 15 (bootselect pins), and apply an
+external pullup/down so that the target is normally running.
+
+## Usage
+
+See the included example. In short:
+
+```arduino
+
+// Create the programmer object
+ESP8266AVRISP avrprog(PORT, RESET_PIN)
+// ... with custom SPI frequency
+ESP8266AVRISP avrprog(PORT, RESET_PIN, 4e6)
+
+// Check current connection state, but don't perform any actions
+AVRISPState_t state = avrprog.update();
+
+// Serve the pending connection, execute STK500 commands
+AVRISPState_t state = avrprog.serve();
+```
+
+### License and Authors
+
+This library started off from the source of ArduinoISP "sketch" included with
+the Arduino IDE:
+
+    ArduinoISP version 04m3
+    Copyright (c) 2008-2011 Randall Bohn
+    If you require a license, see
+        http://www.opensource.org/licenses/bsd-license.php
+
+    Support for TCP on ESP8266
+    Copyright (c) Kiril Zyapkov <kiril@robotev.com>.