[Upgrade] Add info about upgrading From 7.x to 8.0 (#290)

UPGRADE.md

@@ -2,9 +2,48 @@
 
 This document describes the changes needed when upgrading from one version to another.
 
+## Upgrading From 7.x to 8.0
+
+### Step 1: upgrade PHP and Symfony
+
+Minimum required PHP version was raised to 7.1 and Symfony to 4.0.  
+[Rector](https://github.com/rectorphp/rector) can help you with migration.
+
+### Step 2: remove usage of GuzzleEventListenerInterface
+
+The interface `GuzzleEventListenerInterface` was removed.
+Please read the [documentation](https://github.com/8p/EightPointsGuzzleBundle#listening-to-events) in case you want to listen pre/post translation events for specific client.
+
+### Step 3: follow strict typing rules
+
+If your classes are overriding some classes from bundle, then check that all methods are following arguments and return types.
+
+### Step 4: replace usage of EightPointsGuzzleBundlePlugin interface
+
+before:
+```php
+use EightPoints\Bundle\GuzzleBundle\EightPointsGuzzleBundlePlugin;
+
+class Foo implements EightPointsGuzzleBundlePlugin
+{
+
+}
+```
+
+after:
+```php
+use EightPoints\Bundle\GuzzleBundle\PluginInterface;
+
+class Foo implements PluginInterface
+{
+
+}
+```
+
 ## Upgrading From 6.x to 7.0
 
 ### Step 1: change namespace in AppKernel
+
 before:
 ```php
 public function registerBundles()
@@ -30,6 +69,7 @@ public function registerBundles()
 ```
 
 ### Step 2: change config key in app/config/config.yml
+
 before:
 ```yaml
 guzzle:
@@ -47,6 +87,7 @@ eight_points_guzzle:
 ```
 
 ### Step 3: move headers key under options config
+
 before:
 ```yaml
 guzzle:
@@ -69,6 +110,7 @@ eight_points_guzzle:
 ```
 
 ### Step 4: client call
+
 before:
 ```php
 $this->get('guzzle.client.api_crm');
@@ -80,6 +122,7 @@ $this->get('eight_points_guzzle.client.api_crm');
 ```
 
 ### Step 5: event listeners definition
+
 before:
 ```xml
 <service id="listenerID" class="Your\ListenerClass\That\Implements\GuzzleEventListenerInterface">  
@@ -95,6 +138,7 @@ after:
 ```
 
 ### Step 6: if you have created any services, you should change name
+
 before:
 ```xml
 <argument type="service" id="guzzle.client.xyz" />