Add library readme

Author: mladenilic

arduino/README.md

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+# Arduino Cereal Library
+
+Arduino library for tracking variables in [Cereal monitor ](../).
+
+### Install
+
+Download and manually install to your Arduino libraries. Read *Manual Installation* section on Arduino's [library installation instruction](https://www.arduino.cc/en/Guide/Libraries) page.
+
+### Usage
+
+Cereal monitor supports three different types of variables: *basic* variable, *range* and a *flag* variable.
+
+For basic variable output, use:
+```c++
+Cereal.varaible(name, value);
+```
+
+For range variable outputs use: 
+```c++
+Cereal.range(name, value, from, to);
+Cereal.analog(name, value);
+```
+
+For flag variable output use:
+```c++
+Cereal.digital(name, value);
+``` 
+
+### Example
+
+```c++
+#include "Cereal.h"
+
+#define STATUS_PIN 3
+#define THROTTLE_PIN A2
+
+void setup() {
+  // Set the baud rate for serial data transmission 
+  Serial.begin(9600);
+  
+  pinMode(STATUS_PIN, INPUT);
+  pinMode(THROTTLE_PIN, INPUT);
+   
+  // Serial output shown in [Monitor] window
+  Serial.println("Setup done...");
+}
+
+void loop() {
+  int rnd = random(10, 20);
+  int throttle = analogRead(THROTTLE_PIN);
+  int status = digitalRead(STATUS_PIN);
+  
+  // Basic variable type output
+  Cereal.variable("Random", rnd);
+  
+  // Range variable type output
+  Cereal.analog("Throttle", throttle);
+  
+  // Flag variable type output
+  Cereal.digital("Status", status);
+}
+
+```
+
+Running this examples gives the following output:
+
+![image](../images/demo.gif)