Update tools/gn/README.md.
Update tools/gn/README.md.

This patch updates the GN README to reflect the current
state of the migration, and also fixes the links to the
GN docs so that they work correctly (working around
https://code.google.com/p/gitiles/issues/detail?id=82).

R=andybons@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1393863003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#352994}

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