How-to-deprecate.md

 So you want to deprecate something in OTB? Follow the guide:
 
-#### A C++ class
+#### A C++ class (WIP)
 
 In the doxygen documentation of the class, add the `\deprecated` keyword.
 
 In the class declaration, use the `[[deprecated]]` attribute. If the class declaration already has an export macro, use the macro `_DEPRECATED_EXPORT`.
 
-#### A method of a C++ class
+#### A method of a C++ class (WIP)
 
 Use the `[[deprecated]]` attribute.
 
-#### A part of the Python API
+#### A part of the Python API (WIP)
 
-TODO
 
-#### An OTB application
+#### An OTB application (WIP)
 
 Add the tag deprecated:
 
@@ -23,8 +22,44 @@ AddDocTag(Tags::Deprecated);
 ```
 
 #### An OTB module
+An OTB module can be deprecated thanks to the `otb_module` macro in the `otb-module.cmake`. One just need to add the key word `DEPRECATED` the arguments.  
+Here is an example of th `otb-module.cmake` of a deprecated module : 
+```CMake
+set(DOCUMENTATION "Some documentation")
+otb_module(OTBDeprecatedModuleName
+  DEPENDS
+    OTBSomeDependencies
 
-Add the keyword `DEPRECATED` in the call to `otb_module()` macro in file `otb-module.cmake`
+  TEST_DEPENDS
+    OTBTestKernel
+    OTBSomeOtherDependenciesForTest
+  
+  DEPRECATED
+
+  DESCRIPTION
+    "${DOCUMENTATION}"
+)
+```
+
+One need to make sure that the deprecated module is not a mandatory dependency of any other module. One can still allow the module to be use thanks to the `OPTIONAL_DEPENDS` key word :
+```CMake
+set(DOCUMENTATION "Some documentation")
+otb_module(OTBDModuleName
+  DEPENDS
+    OTBSomeDependencies
+  
+  OPTIONAL_DEPENDS
+    OTBSomeOptionalDependencies
+    OTBSomeDeprecatedModule
+
+  TEST_DEPENDS
+    OTBTestKernel
+    OTBSomeOtherDependenciesForTest
+
+  DESCRIPTION
+    "${DOCUMENTATION}"
+)
+```
 
 #### How to disable deprecated features when `OTB_USE_DEPRECATED=OFF`?