Add a comment for the VPython dep

R=machenbach@chromium.org

Bug: chromium:777345
Change-Id: I54e8121ada8590d082bcb172668a82e00b5cf248
Reviewed-on: https://chromium-review.googlesource.com/737632
Commit-Queue: Sergiy Byelozyorov <sergiyb@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Sergiy Byelozyorov <sergiyb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48925}

.vpython

@@ -24,6 +24,8 @@
 
 python_version: "2.7"
 
+# Needed by third_party/catapult/devil/devil, which is imported by
+# build/android/test_runner.py when running performance tests.
 wheel: <
   name: "infra/python/wheels/psutil/${vpython_platform}"
   version: "version:5.2.2"