Add note and link to MM-Jekyll 3 branch.

Author: mmistakes

README.md

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 **[Minimal Mistakes](http://mmistakes.github.io/minimal-mistakes)** is a two column responsive Jekyll theme perfect for powering your GitHub hosted blog.
 
+**Jekyll 3 Update:** A version of Minimal Mistakes compatible with Jekyll 3 can be found in the [`jekyll3` branch](https://github.com/mmistakes/minimal-mistakes/tree/jekyll3). GitHub Pages is [locked at version 2.4](https://pages.github.com/versions/) so some keep that in mind if you're trying to use 3.0 features that aren't supported there yet.
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 ## Minimal Mistakes is all about:
 
 * Responsive templates. Looking good on mobile, tablet, and desktop.