How-to-deprecate.md

@@ -2,10 +2,26 @@ So you want to deprecate something in OTB? Follow the guide:
 
 ## A C++ class
 
+Use the `[[deprecated]]` attribute.
+
+If the class declaration already has an export macro, use the macro `_DEPRECATED_EXPORT`.
+
 ## A method of a C++ class
 
+Use the `[[deprecated]]` attribute.
+
 ## A part of the Python API
 
+TODO
+
 ## An OTB application
 
+Add the tag deprecated:
+
+```
+AddDocTag(Tags::Deprecated);
+```
+
 ## An OTB module
+
+TODO
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