add optional spellchecking support to docs makefile

docs/Makefile

@@ -87,3 +87,8 @@ doctest:
 	$(SPHINXBUILD) -b doctest $(ALLSPHINXOPTS) $(BUILDDIR)/doctest
 	@echo "Testing of doctests in the sources finished, look at the " \
 	      "results in $(BUILDDIR)/doctest/output.txt."
+
+spelling:
+	$(SPHINXBUILD) -b spelling $(ALLSPHINXOPTS) $(BUILDDIR)/doctest
+	@echo "Spell checking of docs is finished.the"
+

docs/conf.py

@@ -24,6 +24,14 @@ import sys, os
 # coming with Sphinx (named 'sphinx.ext.*') or your custom ones.
 extensions = ['sphinx.ext.autodoc', 'sphinx.ext.doctest', 'sphinx.ext.coverage']
 
+try:
+    import sphinxcontrib.spelling
+except ImportError:
+    pass
+else:
+    extensions.append("sphinxcontrib.spelling")
+
+
 # Add any paths that contain templates here, relative to this directory.
 templates_path = ['_templates']