Create README.md

Author: ndomx

README.md

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+# MCP23017-Arduino-Library
+
+![release badge](https://img.shields.io/github/v/release/ndomx/MCP23017-Arduino-Library?style=flat-square)
+![license badge](https://img.shields.io/github/license/ndomx/MCP23017-Arduino-Library?style=flat-square)
+
+
+This is a library to control MCP23017 I2C modules (product page [here](https://www.microchip.com/en-us/product/mcp23017)). The library provides full control of the module using familiar functions like `digitalWrite`, `digitalRead` and `pinMode`, as well as other functionalities.
+
+> Note: The library does not support interrupts yet
+
+## Usage
+Begin by including the library files and declaring an instance of the `MCP23017` class.
+~~~c++
+#include <mcp23017.h>
+
+#define MCP23017_ADDR (0x07) // I2C address of the module
+MCP23017 mcp(MCP23017_ADDR);
+~~~
+
+Every instance of the `MCP23017` class must be initialized after created. This will setup the module so it's compatible with the library, and initializes the `Wire` library if needed. To do this, just call the `start()` function inside `setup()` or `loop()`
+
+After the instance is initialized, Arduino functions can be used to configure the additional I/O ports. These ports will be labeled `GPAn` for ports in the _A_ bank, and `GPBn` for ports in the _B_ bank (`0 <= n < 8`). Below you'll find a basic example, and there are more examples in the [examples folder](https://github.com/ndomx/MCP23017-Arduino-Library/tree/master/examples).
+~~~c++
+// Blink a LED
+
+#include <mcp23017.h>
+
+#define MCP_ADDR (0)
+
+MCP23017 mcp(MCP_ADDR);
+
+void setup(void)
+{
+    mcp.start();
+    mcp.pinMode(GPB7, OUTPUT);
+}
+
+void loop(void)
+{
+    mcp.digitalWrite(GPB7, HIGH);
+    delay(250);
+    mcp.digitalWrite(GPB7, LOW);
+    delay(250);
+}
+~~~