added tutorial for pullup

tut-pullup.md

@@ -0,0 +1,49 @@
+# Pull- Pins
+
+Suppose a pin is configures as an input. If nothing is connected to the pin and the program tries to read the state of the pin, it would need be in a 'floating' state i.e an unknown state. To prevent this, a pull-up or a pull-down state is defined. They are often used in the case of Buttons and Switches.
+
+Pins 2-7 on both the Ports are available for interrupts.
+
+Take a look at the following circuit. The code example given below turns on the Blue LED of the Tessel module when the pushbutton is pressed and turns off the Blue LED when the pushbutton is released.
+
+---Link to the fritzing diagram---
+
+```js
+var tessel= require('tessel'); // Import Tessel
+
+var pin = tessel.port.A.pin[2]; // Select pin 2 on port A
+
+var pullType = "pullup"; // Set the mode of `pin.pull to pullup`
+
+var led = tessel.led[3]; // Blue LED of Tessel
+
+pin.pull(pullType,(error, buffer) => { // Pin 2 pulled high
+
+  if (error){
+    throw error;
+  }
+});
+
+setInterval({
+  pin.read(function(error, number){
+
+      if (error) {
+        throw error;
+      }
+
+      // Pin 2 reads high when the pushbutton is not pressed since it is pulled up
+      console.log(number);
+      if (number == 1){
+        led.off();
+      }
+
+      // Pin 2 reads low when the pushbutton is pressed since its connection with ground gets complete
+      else{
+        led.on();
+      }
+
+  });
+
+}, 500);
+```
+[More Information on Pull-up pins and resistors](https://learn.sparkfun.com/tutorials/pull-up-resistors)