How-to-deprecate.md

@@ -61,6 +61,8 @@ otb_module(OTBDModuleName
 )
 ```
 
+Please make sure that all of the classes in the module have the `ModuleName_DEPRECATED_EXPORT` and the Doxygene tag `\deprecated`.
+
 #### How to disable deprecated features when `OTB_USE_DEPRECATED=OFF`?
 
 For an example, see [this MR](https://gitlab.orfeo-toolbox.org/orfeotoolbox/otb/merge_requests/236/diffs)