Add find_package() to the documentation in the Developers section

src/Cxx/Developers/AlgorithmFilter.md

 ### Description
 
 This example demonstrates how to create a filter that accepts a custom class and returns a custom class. To test that it is working, the class vtkTest simply stores a double named 'Value' that is instantiated to the value of 4.5. Example.cxx instantiates a vtkTest and sets its value to 5.6. The filter adds 1.0 to this value, so the vtkTest that is the output of the vtkTestAlgorithmFilter should contain the value 6.6.
+
+You will need the following in your CMakeLists.txt file:
+
+```cmake
+find_package(VTK
+ COMPONENTS
+    CommonCore
+    CommonDataModel
+    CommonExecutionModel
+    FiltersSources
+    InfovisCore
+)
+```