Update adding_to_third_party to add explicit link to Chrome Eng Review

Change-Id: I567141992ec4a9269068c03a1798f2561dba24f6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1879946
Commit-Queue: Tim van der Lippe <tvanderlippe@chromium.org>
Auto-Submit: Tim van der Lippe <tvanderlippe@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709859}

docs/adding_to_third_party.md

@@ -143,6 +143,8 @@ following sign-offs. Some of these are accessible to Googlers only.
 Non-Googlers can email one of the people in
 [//third_party/OWNERS](../third_party/OWNERS) for help.
 
+* Make sure you have the approval from Chrome Eng Review as mentioned
+  [above](#before-you-start).
 * Get security@chromium.org approval. Email the list with relevant details and
   a link to the CL. Third party code is a hot spot for security vulnerabilities.
   When adding a new package that could potentially carry security risk, make