Add DOCUMENTATION.md (#667)

Adds the C++ `DOCUMENTATION.md`, covering the use of Doxygen for documenting the code. Copied mostly from libcudf.

Contributes to #598

Authors:
  - Mark Harris (https://github.com/harrism)

Approvers:
  - H. Thomson Comer (https://github.com/thomcom)
  - Michael Wang (https://github.com/isVoid)
  - Robert Maynard (https://github.com/robertmaynard)

URL: #667

cpp/CMakeLists.txt

@@ -277,3 +277,27 @@ rapids_export(
   NAMESPACE cuspatial::
   DOCUMENTATION doc_string
 )
+
+
+# ##################################################################################################
+# * build documentation ----------------------------------------------------------------------------
+
+find_package(Doxygen)
+if(Doxygen_FOUND)
+
+# doc targets for cuDF
+add_custom_command(
+  OUTPUT CUSPATIAL_DOXYGEN
+  WORKING_DIRECTORY ${CUSPATIAL_SOURCE_DIR}/doxygen
+  COMMAND ${DOXYGEN_EXECUTABLE} Doxyfile
+  VERBATIM
+  COMMENT "Custom command for building cuspatial doxygen docs."
+)
+
+add_custom_target(
+  docs_cuspatial
+  DEPENDS CUSPATIAL_DOXYGEN
+  COMMENT "Custom command for building cuspatial doxygen docs."
+)
+
+endif()