Dynamically retrieve podCIDR for routing in CONTRIBUTING

The CIDR from which pod IPs are allocated can change depending on
platform and driver used for `minikube`. On my machine, it's
10.244.0.0/24. This changes the instructions to determine the
podCIDR dynamically by talking to kubernetes, rather than
hardcoding it in.

I personally prefer just repeating the command here rather than using
an environment variable, as it means the single line can be just copy
pasted.

CONTRIBUTING.md

@@ -81,14 +81,14 @@ rules on your computer. We will now walk you through the steps to get going:
 
     ```bash
     # Linux
-    sudo ip route add 172.17.0.0/16 via $(minikube ip)
+    sudo ip route add $(kubectl get node minikube -o jsonpath="{.spec.podCIDR}") via $(minikube ip)
     # later on you can undo this with
-    sudo ip route del 172.17.0.0/16
+    sudo ip route del $(kubectl get node minikube -o jsonpath="{.spec.podCIDR}")
 
     # MACOS
-    sudo route -n add -net 172.17.0.0/16 $(minikube ip)
+    sudo route -n add -net $(kubectl get node minikube -o jsonpath="{.spec.podCIDR}") $(minikube ip)
     # later on you can undo this with
-    sudo route delete -net 172.17.0.0
+    sudo route delete -net $(kubectl get node minikube -o jsonpath="{.spec.podCIDR}")
     ```
 
     ### Troubleshooting