Create USAGE.md

Author: nvzard

USAGE.md

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+# Table of Contents
+- [Installation](#installation)
+- [Quick Start](#quick-start)
+- [Usage](#usage)
+
+<a name="installation"></a>
+# Installation
+
+## Prerequisites
+
+- PHP version 5.6 or 7.0
+
+## Install with Composer
+
+Add php-http-client to your `composer.json` file. If you are not using [Composer](http://getcomposer.org), you should be. It's an excellent way to manage dependencies in your PHP application.
+
+```json
+{
+  "require": {
+    "sendgrid/php-http-client": "~3.8"
+  }
+}
+```
+
+Then at the top of your PHP script require the autoloader:
+
+```php
+require __DIR__ . '/vendor/autoload.php';
+```
+
+Then from the command line:
+
+```bash
+composer install
+```
+
+## Install without Composer
+
+You should create directory `lib` in directory of your application and clone to `lib` repositories [php-http-client](https://github.com/sendgrid/php-http-client.git) and [sendgrid-php](https://github.com/sendgrid/sendgrid-php.git):
+
+```
+$ cd /path/to/your/app
+$ mkdir lib
+$ cd lib
+$ git clone https://github.com/sendgrid/php-http-client.git
+$ git clone https://github.com/sendgrid/sendgrid-php.git
+```
+
+In the next step you should create `loader.php`:
+
+```
+$ cd /path/to/your/app
+$ touch loader.php
+```
+
+And add to `loader.php` code below:
+
+```php
+<?php
+
+require_once __DIR__ . '/lib/php-http-client/lib/Client.php';
+require_once __DIR__ . '/lib/php-http-client/lib/Response.php';
+require_once __DIR__ . '/lib/sendgrid-php/lib/SendGrid.php';
+
+```
+
+After it you can use `php-http-client` library in your project:
+
+```php
+<?php
+
+include __DIR__ . '/loader.php';
+
+$client = new SendGrid\Client();
+```
+
+<a name="quick-start"></a>
+# Quick Start
+
+Here is a quick example:
+
+`GET /your/api/{param}/call`
+
+```php
+// include __DIR__ . '/loader.php';
+require 'vendor/autoload.php';
+$global_headers = array('Authorization: Basic XXXXXXX');
+$client = new SendGrid\Client('base_url', $global_headers);
+$response = $client->your()->api()->_($param)->call()->get();
+print $response->statusCode();
+print $response->headers();
+print $response->body();
+```
+
+`POST /your/api/{param}/call` with headers, query parameters and a request body with versioning.
+
+```php
+// include __DIR__ . '/loader.php';
+require 'vendor/autoload.php';
+$global_headers = array('Authorization: Basic XXXXXXX');
+$client = new SendGrid\Client('base_url', $global_headers);
+$query_params = array('hello' => 0, 'world' => 1);
+$request_headers = array('X-Test' => 'test');
+$data = array('some' => 1, 'awesome' => 2, 'data' => 3);
+$response = $client->your()->api()->_($param)->call()->post('data',
+                                                            'query_params',
+                                                            'request_headers');
+print $response->statusCode();
+print $response->headers();
+print $response->body();
+```
+
+<a name="usage"></a>
+# Usage
+
+- [Example Code](https://github.com/sendgrid/php-http-client/tree/master/examples)
+