* Rename `LICENSE.md` to `LICENSE.txt` so that more text editors
  know how to open it.

* Make the build step copy the `LICENSE.txt` file in the `dist/`
  directory so that it gets included in the distribution package.

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Ref https://www.youtube.com/watch?v=hWDSToC9EV8&t=15m15s
    h5bp/html5-boilerplate#1655

Fix h5bp/html5-boilerplate#1655
Close h5bp/html5-boilerplate#1680