Recommend readers to view slsa.dev, not GitHub.

When viewing the page on GitHub, there is now a link telling the reader
that it is best viewed at https://slsa.dev. This does not show up when
rendered via GitHub Pages.

README.md

 # SLSA: Supply-chain Levels for Software Artifacts
 
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+**NOTE: This site is best viewed at https://slsa.dev.**
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 Supply-chain Levels for Software Artifacts (SLSA, pronounced
 _[salsa](https://www.google.com/search?q=how+to+pronounce+salsa)_) is an
 end-to-end framework for ensuring the integrity of software artifacts throughout