doc updated

Author: sujancse

README.md

@@ -1 +1,47 @@
-## A fast and tiny PHP library to export data to csv, excel etc.
+## Overview
+A fast and tiny PHP library to export data to csv, excel etc. 
+The library is based of PHP generator. It used only 20MB memory
+to download 5 Million data. Tested on Laravel 6.
+
+### Installation
+```$xslt
+composer require sujan/exporter
+```
+
+## Usage
+`use Sujan\Exporter\Export
+`
+```$xslt
+$columns = [
+    'id',
+    'name',
+    'email'
+];
+
+$exporter = new Export(
+    User::query(),
+    $columns
+);
+
+$exporter->export();
+```
+
+**For eloquent relation**
+```$xslt
+$columns = [
+    'id',
+    'title',
+    'user' => [ // user is a relation
+        'name'
+    ]
+];
+
+$exporter = new Export(
+    Post::with('user'),
+    $columns
+);
+
+$exporter->export();
+```
+
+Where `user` is the relation name, which is same as is in the `$columns` variable.