[docs] document phar-stub bug in pitfalls

- https://gist.github.com/916005
- http://silex-ru.net/discussion/10/fatal-error-class-silexapplication-not-found

doc/usage.rst

@@ -494,3 +494,14 @@ If you are on Suhosin you will also have to set this:
 .. code-block:: ini
 
     suhosin.executor.include.whitelist = phar
+
+Phar-Stub bug
+~~~~~~~~~~~~~
+
+Some PHP installations have a bug that throws a ``PharException`` when trying
+to include the Phar. It will also tell you that ``Silex\Application`` could not
+be found. A workaround is using the following include line::
+
+    require_once 'phar://'.__DIR__.'/silex.phar/autoload.php';
+
+The exact cause of this issue could not be determined yet.