Added known issue for complex complex example failure.

README.markdown

@@ -73,6 +73,7 @@ Known Issues
 ------------
 
  * Sections don't respect delimiter changes -- `delimiters` example currently fails with an "unclosed section" exception.
+ * Since `complex` example emulates some fancy swizzling available in Ruby, it fails. Need to convert example to PHPisms (In PHP, Mustache class doesn't maintain current context stack -- available context is passed to methods via params).
  * Test coverage is incomplete.