When this README.md is published to pub.dartlang.org, the base URL is
pub.dartlang.org. We need to use absolute URLs in order to get back to GitHub.

Fixes #417

This CL introduces a SkyShell.apk that is separate from the SkyDemo.apk that we
upload to the store to show our demos.  The SkyShell.apk is just an empty shell
that can run Sky applications on Android.

Grammar

Try to clarify initial execution.

Also, stop pushing the SDK to sky_sdk now that this repo exists.