docs: Enable sphinx-autobuild

```bash
cd docs
make
```

to run a webserver that automatically rerenders whenever the rST is
modified.

docs/Makefile

@@ -2,7 +2,7 @@
 #
 
 default:
-	sphinx-autobuild . _build/html/
+	sphinx-autobuild -b html $(ALLSPHINXOPTS) $(BUILDDIR)/html
 
 # You can set these variables from the command line.
 SPHINXOPTS    =

docs/docs-requirements.txt

@@ -1,2 +1,3 @@
 Sphinx==1.7.1
+sphinx-autobuild==0.7.1
 sphinxcontrib-programoutput==0.11