Improve README describing how to add third_party packages to DevTools
Improve README describing how to add third_party packages to DevTools

R=6d0eecbeb7fb9d172f49b4b32481d2466fd89429@chromium.org

Change-Id: I19e08997fb8d38d351ebc42ca6c72b7a2efe3633
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2282343
Commit-Queue: Simon Zünd <38944668e9e2ca98ccd0275b53370f90fa55c113@chromium.org>
Reviewed-by: Peter Marshall <6d0eecbeb7fb9d172f49b4b32481d2466fd89429@chromium.org>
Reviewed-by: Tim van der Lippe <dba8716ee7f8d16236046f74d2167cb94410f6ed@chromium.org>

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