.nojekyll

From GitHub Staff, 2016/11/04

If you're not using Jekyll, you can add a .nojekyll file to the root of your repository to disable Jekyll from building your site. Once you do that, your site should build correctly.

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Reason: GitHub build auto-convert the markdown files and don't serve them.
Ref: https://github.com/blog/572-bypassing-jekyll-on-github-pages