Add note about how to render graphviz into the README

rust/datafusion/README.md

@@ -171,3 +171,24 @@ Below is a checklist of what you need to do to add a new aggregate function to D
   * a new line in `create_aggregate_expr` mapping the built-in to the implementation
   * tests to the function.
 * In [tests/sql.rs](tests/sql.rs), add a new test where the function is called through SQL against well known data and returns the expected result.
+
+## How to display plans graphically
+
+The query plans represented by `LogicalPlan` nodes can be graphically
+rendered using [Graphviz](http://www.graphviz.org/).
+
+To do so, save the output of the `display_graphviz` function to a file.:
+
+```rust
+// Create plan somehow...
+let mut output = File::create("/tmp/plan.dot")?;
+write!(output, "{}", plan.display_graphviz());
+```
+
+Then, use the `dot` command line tool to render it into a file that
+can be displayed. For example, the following command creates a
+`/tmp/plan.pdf` file:
+
+```bash
+dot -Tpdf < /tmp/plan.dot > /tmp/plan.pdf
+```