Add autobuild target for docs makefile

Based on work by Armen Zambrano <armenzg@mozilla.com>

docs/Makefile

@@ -19,7 +19,7 @@ ALLSPHINXOPTS   = -d $(BUILDDIR)/doctrees $(PAPEROPT_$(PAPER)) $(SPHINXOPTS) .
 # the i18n builder cannot share the environment and doctrees with the others
 I18NSPHINXOPTS  = $(PAPEROPT_$(PAPER)) $(SPHINXOPTS) .
 
-.PHONY: help clean html dirhtml singlehtml pickle json htmlhelp qthelp devhelp epub latex latexpdf text man changes linkcheck doctest gettext
+.PHONY: help clean html dirhtml singlehtml pickle json htmlhelp qthelp devhelp epub latex latexpdf text man changes linkcheck doctest gettext livehtml
 
 help:
 	@echo "Please use \`make <target>' where <target> is one of"
@@ -175,3 +175,7 @@ pseudoxml:
 	$(SPHINXBUILD) -b pseudoxml $(ALLSPHINXOPTS) $(BUILDDIR)/pseudoxml
 	@echo
 	@echo "Build finished. The pseudo-XML files are in $(BUILDDIR)/pseudoxml."
+
+livehtml:
+	make html
+	sphinx-autobuild -b html $(ALLSPHINXOPTS) $(BUILDDIR)/html

requirements/docs.txt

@@ -7,5 +7,6 @@
 # would not help those who wish to build the docs locally.
 
 Sphinx
+sphinx-autobuild
 sphinx-rtd-theme
 sphinxcontrib-httpdomain