Update Importer.hpp (#6235)

- Add doc for out-of-memory exception
- closes https://github.com/assimp/assimp/issues/5173

include/assimp/Importer.hpp

@@ -108,6 +108,10 @@ namespace Assimp {
 * allocations and may take some time, so it's better to reuse them as often as
 * possible.
 *
+* If you want to let assimp deal with OutOfMemory-exception make sure that
+* ASSIMP_CATCH_GLOBAL_EXCEPTIONS is set.
+* If this is not the case you need to catch the exception by yourself.
+*
 * If you need the Importer to do custom file handling to access the files,
 * implement IOSystem and IOStream and supply an instance of your custom
 * IOSystem implementation by calling SetIOHandler() before calling ReadFile().