Document the lightweight cleaning mode

doc/implementation_notes.md

 Implementation notes
 ====================
 
+Lightweight cleaning mode
+-------------------------
+
+Due to *popular* request, MAT2 is providing a *lightweight* cleaning mode,
+that only cleans the superficial metadata of your file, but not
+the ones that might be in **embeded** resources. Like for example,
+images in a PDF or an office document.
+
 Symlink attacks
 ---------------