fix(#13,doc): code to plot workflow diagram in intro

Author: ankostis

docs/source/index.rst

@@ -69,6 +69,16 @@ Here's a Python script with an example GraphKit computation graph that produces
 
 As you can see, any function can be used as an operation in GraphKit, even ones imported from system modules!
 
+As a debug aid, you may plot the workflow with this code::
+
+   graph.net.plot(show=True)
+
+> NOTE: `graphviz` must be in your PATH and `matplotlib` installed.
+> You may install both when installing *graphkit* with its `plot` extras::
+>
+>     pip install graphkit[plot]
+
+
 License
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