add 'tox -e docs', to check+render .rst files

tox.ini

@@ -30,3 +30,18 @@ commands =
          python src/allmydata/test/check_memory.py download-GET
          python src/allmydata/test/check_memory.py download-GET-slow
          python src/allmydata/test/check_memory.py receive
+
+# Use 'tox -e docs' to check formatting and cross-references in docs .rst
+# files. The published docs are built by code run over at readthedocs.org,
+# which does not use this target (but does something similar).
+#
+# If you have "sphinx" installed in your virtualenv, you can just do "make -C
+# docs html", or "cd docs; make html".
+#
+# You can also open docs/_build/html/index.html to see the rendered docs in
+# your web browser.
+
+[testenv:docs]
+deps = sphinx
+commands =
+         sphinx-build -b html -d {toxinidir}/docs/_build/doctrees {toxinidir}/docs {toxinidir}/docs/_build/html