update README

Change the calls from "addWarning" and "addError" to the new pattern "warning" and "error"

README.mdown

@@ -37,8 +37,8 @@ $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');
+$log->warning('Foo');
+$log->error('Bar');
 ```
 
 ## Documentation