Add @platformio as alternative option to use ESP8266 core for Arduino

Hi @igrr,

Would be thankful to place some information about [PlatformIO[(http://platformio.org) as alternative cross-platform builder to use ESP8266 core for Arduino on the popular host OS: Mac OS X, Windows, Linux 32/64, Linux ARM (like Raspberry Pi, BeagleBone, CubieBoard).

Don't hesitate to modify this PR.

Thanks in advance!

README.md

@@ -48,6 +48,69 @@ python get.py
 ```
 - Restart Arduino
 
+### Using PlatformIO
+
+[PlatformIO](http://platformio.org) is a cross-platform code-builder and library manager for embedded development with no external dependencies and support for Espressif platform. Works on the popular host OS: Mac OS X, Windows, Linux 32/64, Linux ARM (like Raspberry Pi, BeagleBone, CubieBoard).
+
+- Install [PlatformIO](http://platformio.org)
+- Initialise new project
+```bash
+# 
+# Create empty directory
+#
+mkdir myproject
+cd myproject
+
+# 
+# Find type of supported boards
+#
+platformio boards espressif
+
+# Platform: espressif
+# --------------------------------------------------------------------------------------------------------
+# Type                  MCU            Frequency  Flash   RAM    Name
+# --------------------------------------------------------------------------------------------------------
+# esp01                 esp8266        80Mhz     512Kb   32Kb   Espressif ESP8266 ESP-01 board
+# esp01_1m              esp8266        80Mhz     1024Kb  32Kb   Espressif ESP8266 ESP-01-1MB board
+# esp12e                esp8266        80Mhz     4096Kb  32Kb   Espressif ESP8266 ESP-12E board (NodeMCU)
+# ...
+
+#
+# Initialise base project
+#
+platformio init --board %TYPE%(see above)
+
+# The next files/directories will be created in myproject
+# platformio.ini - Project Configuration File. |-> PLEASE EDIT ME <-|
+# src - Put your source files here
+# lib - Put here project specific (private) libraries
+# Do you want to continue? [y/N]: Y
+```
+- Place your source code to `src` directory
+- Build/Upload project
+```
+# process/build project
+platformio run
+
+# build+upload firmware
+platformio run --target upload
+
+# build+upload firmware via OTA
+platformio run --target upload --upload-port IP_ADDRESS_HERE
+```
+
+### IDE Integration
+In addition, PlatformIO [can be integrated into the popular IDEs](http://docs.platformio.org/en/latest/ide.html). For example, initialise project for Espressif ESP8266 ESP-01 board and Eclipse IDE
+```
+platformio init --board esp01 --ide eclipse
+```
+Then [import project](http://docs.platformio.org/en/latest/ide/eclipse.html) using `Eclipse Menu: File > Import... > General > Existing Projects into Workspace`.
+
+### Demo of OTA firmware uploading
+[![PlatformIO and OTA firmware uploading to Espressif ESP8266 ESP-01](http://img.youtube.com/vi/W8wWjvQ8ZQs/0.jpg)](http://www.youtube.com/watch?v=W8wWjvQ8ZQs "PlatformIO and OTA firmware uploading to Espressif ESP8266 ESP-01")
+
+### Documentation
+
 Documentation for latest development version:
 
 - [Reference](doc/reference.md)