doc: Add contributing guide to build

buildsupport/macros_and_definitions.cmake

@@ -827,6 +827,7 @@ macro(process_options)
     sphinx_target(manual)
     sphinx_target(models)
     sphinx_target(tutorial)
+    sphinx_target(contributing)
   endif()
   # end of checking for documentation dependencies and creating targets
 

ns3

@@ -210,7 +210,7 @@ def parse_args(argv):
                                         help='Try "./ns3 docs --help" for more documentation options')
     parser_docs.add_argument('docs',
                              help='Build project documentation',
-                             choices=["manual", "models", "tutorial", "sphinx", "doxygen-no-build", "doxygen", "all"],
+                             choices=["manual", "models", "tutorial", "contributing", "sphinx", "doxygen-no-build", "doxygen", "all"],
                              action="store", type=str, default=None)
 
     parser.add_argument('-j', '--jobs',
@@ -721,6 +721,7 @@ def build_step(args,
                                   "manual",
                                   "models",
                                   "tutorial",
+                                  "contributing",
                                   "install",
                                   "uninstall",
                                   ]