Add XR instrumentation test TL;DR

Adds a TL;DR to the XR instrumentation test documentation on how to
locally reproduce most VR issues since many VR breakages at this point
are caused by non-VR CLs, and their authors often need to ask how to
reproduce the issue.

Change-Id: I2a4585969a859cb9c9256c8cfa0715d9cc07b71c
Reviewed-on: https://chromium-review.googlesource.com/c/1352488Reviewed-by: Bill Orr <billorr@chromium.org>
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612326}

chrome/android/javatests/src/org/chromium/chrome/browser/vr/README.md

 # XR Instrumentation Tests
 
+## TL;DR For Most Local Repros
+
+1. Get a rooted Pixel device of some sort.
+2. Make sure "VR Services" is up to date in the Playstore.
+3. Run `ninja -C out/Debug chrome_public_test_vr_apk
+        && out/Debug/bin/run_chrome_public_test_vr_apk
+        --num-retries=0
+        --shared-prefs-file=//chrome/android/shared_preference_files/test/vr_ddview_skipdon_setupcomplete.json
+        --test-filter=<failing test case>`
+
 ## Introduction
 
 This directory contains all the Java-side infrastructure for running