[Coverage] Link to coverage.py to code_coverage.md

Link to coverage.py to code_coverage.md

Bug: 833912
Change-Id: I089bbef64424004f543823d663dad00b3a275fbe
Reviewed-on: https://chromium-review.googlesource.com/1017239Reviewed-by: Max Moroz <mmoroz@chromium.org>
Commit-Queue: Max Moroz <mmoroz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551809}

tools/code_coverage/coverage.py

@@ -53,6 +53,9 @@
              want to see the coverage on corpus and don't want to fuzz at all.
 
   For more options, please refer to tools/code_coverage/coverage.py -h.
+
+  For an overview of how code coverage works in Chromium, please refer to
+  https://chromium.googlesource.com/chromium/src/+/master/docs/code_coverage.md
 """
 
 from __future__ import print_function