Merge pull request #35 from Johnlon/master

Documentation improvement

Author: ladyada

README.md

@@ -13,3 +13,27 @@ All text above must be included in any redistribution
 To download. click the DOWNLOADS button in the top right corner, rename the uncompressed folder Adafruit_MCP23017. Check that the Adafruit_MCP23017 folder contains Adafruit_MCP23017.cpp and Adafruit_MCP23017.h
 
 Place the Adafruit_MCP23017 library folder your <arduinosketchfolder>/libraries/ folder. You may need to create the libraries subfolder if its your first library. Restart the IDE.
+
+Pin Addressing
+==============
+
+When using single pin operations such as _pinMode(pinId, dir)_ or _digitalRead(pinId)_  or _digitalWrite(pinId, val)_ then the pins are addressed using the ID's below. For example, for set the mode of _GPB0_ then use _pinMode(8, ...)_.
+
+Physial Pin #| Pin Name | Pin ID
+----|------|------------------------------
+21 | GPA0 | 0
+22 | GPA1 | 1
+23 | GPA2 | 2
+24 | GPA3 | 3
+25 | GPA4 | 4
+26 | GPA5 | 5
+27 | GPA6 | 6
+28 | GPA7 | 7
+1 | GPB0 |  8
+2 | GPB1 |  9
+3 | GPB2 | 10
+4 | GPB3 | 11
+5 | GPB4 | 12
+6 | GPB5 | 13
+7 | GPB6 | 14
+8 | GPB7 | 15