Quickstart: Suggest plotting the watchpoint as a next step (precice#752)

Author: k4rtikx

quickstart/README.md

@@ -125,6 +125,14 @@ You may be curious what displacements OpenFOAM received from the rigid body solv
 
 ![result](images/quickstart-result.png)
 
+Additionally, as we defined a watchpoint on the `Solid` participant at the flap tip (see `precice-config.xml`), we can plot it with gnuplot:
+
+```bash
+./plotDisplacement.sh
+```
+
+The resulting plot shows the y-displacement of the flap tip over time.
+
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