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* Rather than guiding devs to getting the code and preparing a CL, ask
them to first obtain Chrome Eng Review. This will make sure devs
consider the checkout size and build times up front
* Add a section about adding new Node packages. This was previously
undocumented and not clear for new contributors.

Bug: 1012733
Change-Id: I07317d40adb1130b8a1acb6de8b7bd547a09e876
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1867149Reviewed-by: Mathias Bynens <mathias@chromium.org>
Commit-Queue: Tim van der Lippe <tvanderlippe@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707330}