outline the requirement for php-http via composer (#130)

* outline the requirement for http-php via composer

* update reference to setting up request adapter

Author: styks1987

Diff for: README.md

@@ -21,6 +21,13 @@ The recommended way to install the SparkPost PHP Library is through composer.
 curl -sS https://getcomposer.org/installer | php
 ```
 
+Sparkpost requires php-http client (see [Setting up a Request Adapter](#setting-up-a-request-adapter)). There are several [providers](https://packagist.org/providers/php-http/client-implementation) available. If you were using guzzle6 your install might look like this.
+
+```
+composer require guzzlehttp/guzzle
+composer require php-http/guzzle6-adapter
+```
+
 Next, run the Composer command to install the SparkPost PHP Library:
 
 ```
@@ -291,4 +298,4 @@ An exception will be thrown in two cases: there is a problem with the request or
 
 
 ### Contributing
-See [contributing](https://github.com/SparkPost/php-sparkpost/blob/master/CONTRIBUTING.md).
+See [contributing](https://github.com/SparkPost/php-sparkpost/blob/master/CONTRIBUTING.md).