docs: Add section to display Avocado module dependencies.

Add section inside "Development Tips" to include a graph
of module dependencies.
Signed-off-by: NRudá Moura <rmoura@redhat.com>

docs/Makefile

@@ -151,3 +151,13 @@ doctest:
 	$(SPHINXBUILD) -b doctest $(ALLSPHINXOPTS) $(BUILDDIR)/doctest
 	@echo "Testing of doctests in the sources finished, look at the " \
 	      "results in $(BUILDDIR)/doctest/output.txt."
+
+
+# Requires:
+# - snakefood (pip install snakefood)
+# - graphviz (yum install graphviz / apt-get install graphviz)
+modules.png:
+	cd .. && sfood --internal avocado | sfood-graph --remove-extensions | \
+		grep -v 'size = "8,10"' | sed 's/fontsize=7/fontsize=16/' | \
+		grep -v 'avocado.core.plugins' | grep -v 'avocado.restclient' | \
+		dot -Tpng -o docs/source/modules.png

docs/source/DevelopmentTips.rst

@@ -150,3 +150,7 @@ My usual workflow is:
 
 This way you always have all the files present and you can easily resume
 your work.
+
+Module Dependencies
+===================
+.. figure:: modules.png