add a link to the source of the echo client's main.go (#17343)
add a link to the source of the echo client's main.go (#17343)

to check the configuration options of client, there is no need to
run the image, the options are in the code

There are no instructions how to run the test pod, what is the image,
so it is much simpler for the reader just to check the source code.
smawson/istio,geeknoid/istio,istio/istio,smawson/istio,istio/istio,geeknoid/istio,smawson/istio,smawson/istio,geeknoid/istio,geeknoid/istio,geeknoid/istio,geeknoid/istio,istio/istio,smawson/istio,istio/istio,istio/istio,smawson/istio