Add changelog file that points to GitHub Releases & our blog

Refs twbs#16739
[skip sauce]
[skip validator]

Author: cvrebert

CHANGELOG.md

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+Bootstrap uses [GitHub's Releases feature](https://github.com/blog/1547-release-your-software) for its changelogs.
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+See [the Releases section of our GitHub project](https://github.com/twbs/bootstrap/releases) for changelogs for each release version of Bootstrap.
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+Release announcement posts on [the official Bootstrap blog](http://blog.getbootstrap.com) contain summaries of the most noteworthy changes made in each release.