readme: add information about new predis adapter

Author: mtk3d

README.md

@@ -84,6 +84,19 @@ Change the Redis options (the example shows the defaults):
 );
 ```
 
+Using the Predis storage (requires `predis/predis`):
+```php
+$registry = new CollectorRegistry(new \Prometheus\Storage\Predis());
+```
+
+Or with an existing connection:
+```php
+$client = new \Predis\Client(['host' => '127.0.0.1']);
+$registry = new CollectorRegistry(\Prometheus\Storage\Predis::fromExistingConnection($client));
+```
+
+> **Note:** `Redis::setPrefix()` and `Predis::setPrefix()` share the same prefix. Using both adapters with different prefixes in the same application is not supported.
+
 Using the InMemory storage:
 ```php
 $registry = new CollectorRegistry(new InMemory());