updated README

[beardedN5rd]

Tried to checkout and build on a clean ubuntu. In comparison with the tools needed make, unzip and g++ were missing.

[grpc-kokoro]

Thanks for your pull request. The automated tests will run as soon as one of the admins verifies this change is ok to test.

[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).

📝 Please visit https://cla.developers.google.com/ to sign.

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

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[xfxyjwf]

Thanks. Could you sign google CLA as the commented by the googlebot?

[beardedN5rd]

I signed it!

[googlebot]

CLAs look good, thanks!

[xfxyjwf]

Thanks!