stub out flexbox layout docs

docs/_data/nav.yml

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         - title: Viewports
         - title: Print
         - title: Test cases
+    - title: Flexbox
 
 - title: Components
   pages:

docs/layout/flexbox.md

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+---
+layout: page
+title: Flexbox
+---
+
+Flexbox support has finally come to Bootstrap. Opt-in to the new CSS layout styles with the flick of a variable or the swap of a stylesheet.
+
+## What's included
+
+Flexbox support is available for a number of Bootstrap's components:
+
+- The entire grid system (mixins and predefined classes), which switch from `float`s to `display: flex;`.
+- Input groups, which move from `display: table;` to `display: flex;`.
+- The media component moves from `display: table;` and a number of hacky styles to a simple `display: flex;`.
+
+Vendor prefixes are provided in our compiled CSS with Autoprefixer via Grunt.
+
+## Why flexbox?
+
+In a nutshell, flexbix provides simpler and more flexible layout options in CSS. More specifically, it provides:
+
+- Easy vertical alignment of content within a parent element.
+- Easy reordering of content across devices and screen resolutions with the help of media queries.
+- Easy CSS-only equal height columns for your grid-based layouts.
+
+All these things are possible outside flexbox, but typically require extra hacks and workarounds to do right.
+
+## How it works
+
+If you're familiar with modifying variables in Sass—or any other CSS preprocessor—you'll be right at home to move into flexbox mode.
+
+1. Open the `_variables.scss` file and find the `$enable-flex` variable.
+2. Change it from `false` to `true`.
+3. Recompile, and done!
+
+Alternatively, if you don't need the source Sass files, you may swap the default Bootstrap compiled CSS with the compiled flexbox variation. [Head to the download page](/getting-started/download) for more information.
+
+## Browser support
+
+Enabling flexbox means **reduced browser and device support:**
+
+- Internet Explorer 9 and below do not support flexbox.
+- Internet Explorer 10 has a few known quirks.
+
+Please be extra conscious of your user base when enabling flexbox in your project.