add godoc link

[kazuminn]

To make it easier to follow the link to doc

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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Once you've signed, please reply here (e.g. I signed it!) and we'll verify. Thanks.

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[kazuminn]

I sined it!

[googlebot]

CLAs look good, thanks!