Add GitHub PHP syntax highlighting to example

README.mdown

@@ -6,16 +6,20 @@ Monolog - Logging for PHP 5.3
 Usage
 -----
 
-    use Monolog\Logger;
-    use Monolog\Handler\StreamHandler;
+```php
+<?php
 
-    // create a log channel
-    $log = new Logger('name');
-    $log->pushHandler(new StreamHandler('path/to/your.log', Logger::WARNING));
+use Monolog\Logger;
+use Monolog\Handler\StreamHandler;
 
-    // add records to the log
-    $log->addWarning('Foo');
-    $log->addError('Bar');
+// create a log channel
+$log = new Logger('name');
+$log->pushHandler(new StreamHandler('path/to/your.log', Logger::WARNING));
+
+// add records to the log
+$log->addWarning('Foo');
+$log->addError('Bar');
+```
 
 Core Concepts
 -------------