update install instructions

Author: DomasWEB

README.md

@@ -6,7 +6,7 @@ You can find more examples and information about MailerLite API v2 here: [http:/
 
 ## Getting started
 
-In order to use this library you need to have at least PHP 5.6 version.
+In order to use this library you need to have at least PHP 7.1 version.
 
 There are two ways to use MailerLite PHP SDK:
 
@@ -20,6 +20,13 @@ Then you will need to run this simple command using CLI:
 composer require mailerlite/mailerlite-api-v2-php-sdk
 ```
 
+This library is built atop of [PSR-7](https://www.php-fig.org/psr/psr-7/) and
+[PSR-18](https://www.php-fig.org/psr/psr-18/). If you are receiving `Http\Discovery\Exception\DiscoveryFailedException` exception you will need to run:
+
+```bash
+composer require php-http/guzzle6-adapter
+```
+
 ##### Manual (preferable for shared hostings)
 
 This way is preferable only if you are using shared hosting and do not have a possibility to use Composer. You will need to download [this archive](https://bit.ly/32jmi7M)(v0.2.3), extract it and place its contents in root folder of your project. The next step is the same as using Composer, you will need to require `vendor/autoload.php` file in your index.php and lets dive in!