doc: add a JQ example

[pozil]

Having a CLI that generates JSON is powerful for automating certain tasks (CI and more). You can easily parse the JSON output with the JQ utility so I think that it's interesting to show a small script example in the readme that demonstrates this:

#!/bin/bash

json=$(sfdx flow:scan)
flowCount=$(echo $json | jq '.summary.totalFlows')
errorCount=$(echo $json | jq '.summary.totalResults')
echo "Scanned $flowCount flows"
echo "Found $errorCount errors"