Update README.md

[gobijan]

Add link to package documentation

[google-cla]

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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[karolpiotrowicz]

Thanks for the contribution. I'm going to close this PR as the README file evolved.

I noticed the automated check for the Contributor License Agreement (CLA) failed. We need that signed for any merged contributions. Please make sure to sign the CLA before making future submissions—we'd love to see more PRs from you!