Add a readme for the network tester.

BUG=None

Review-Url: https://codereview.webrtc.org/3011653003
Cr-Original-Commit-Position: refs/heads/master@{#19631}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 137cd1d788ea2630d5996d8e76cdcae410f014fb
1 file changed