Document the phar installation

Author: jakzal

Diff for: README.md

@@ -11,10 +11,27 @@ to any property tagged with `@inject`.
 
 ## Installation
 
+### Composer
+
 ```bash
 composer require --dev zalas/phpunit-injector
 ```
 
+### Phar
+
+The extension is also distributed as a PHAR, which can be downloaded together with its pubkey from the most recent
+[Github Release](https://github.com/jakzal/phpunit-injector/releases).
+
+Put the extension and its key in your PHPUnit extensions directory.
+Remember to instruct PHPUnit to load extensions in your `phpunit.xml`:
+
+```xml
+<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/7.0/phpunit.xsd"
+         extensionsDirectory="tools/phpunit.d"
+>
+</phpunit>
+```
+
 ## Configuration
 
 Enable the service injector listener