Add the sky sdk README back.
Add the sky sdk README back.

Was removed in https://github.com/domokit/mojo/commit/1a396394dd951450f0ff5c2da81dced0ca18e8c8

R=eseidel@chromium.org, johnmccutchan@google.com

Review URL: https://codereview.chromium.org/1159013011

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