Add documentation for GN-Typescript integration
Add documentation for GN-Typescript integration

Now that we have a stable implementation of our GN-Typescript integration,
add a README with basic usage and implementation details of it.

R=8915f5d5c39a08c5a3e8a8d7314756e4580816f9@chromium.org,993fcadce4d04090da2fefd557a0995e7966c8d5@chromium.org

Bug: 1011811
Change-Id: I25a11b3483d306ae359c4bf11ecc34fcbff8fd6e
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2112367
Reviewed-by: Paul Lewis <8915f5d5c39a08c5a3e8a8d7314756e4580816f9@chromium.org>
Reviewed-by: Jack Franklin <993fcadce4d04090da2fefd557a0995e7966c8d5@chromium.org>
Commit-Queue: Tim van der Lippe <dba8716ee7f8d16236046f74d2167cb94410f6ed@chromium.org>

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