DOC: add show to makefile

Author: tacaswell

docs/Makefile

@@ -18,3 +18,5 @@ help:
 # "make mode" option.  $(O) is meant as a shortcut for $(SPHINXOPTS).
 %: Makefile
 	@$(SPHINXBUILD) -M $@ "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)
+show:
+	@python -c "import webbrowser; webbrowser.open_new_tab('file://$(shell pwd)/build/html/index.html')"