Add introduction section to the styleguide

Author: onurkerimov

demo/README.md

@@ -1 +1,14 @@
-this is readme
+**react-date-range** can be installed with **npm**, by the following command:
+
+```bash static 
+npm install react-date-range
+```
+
+First, styles and theme file must be imported:
+
+```js static 
+import 'react-date-range/dist/styles.css'; // main css file
+import 'react-date-range/dist/theme/default.css'; // theme css file
+```
+
+Then, import one of the 4 standalone components: [DateRange](#daterange), [DateRangePicker](#daterangepicker), [Calendar](#calendar), [DefinedRange](#definedrange)