Update README.md

Author: nikolas

README.md

@@ -1,2 +1,14 @@
 # react-range-step-input
-An <input type=range> that steps up and down on click
+An <input type=range> that steps up and down on click.
+
+## Installation
+
+    npm install react-range-step-input
+    
+## Usage
+
+    import RangeStepInput from 'react-range-step-input';
+
+In your `render()` method:
+
+    <RangeStepInput />