Add docs regarding postcss-flexbugs-fixes dependency

Author: cvrebert

docs/getting-started/download.md

@@ -21,7 +21,7 @@ Download just the compiled and minified CSS and JavaScript. Doesn't include any
   <div class="col-sm-6">
 {% markdown %}
 ### Source files
-Download everything: source Sass, JavaScript, and documentation files. **Requires a Sass compiler, [Autoprefixer](https://github.com/postcss/autoprefixer), and [some setup]({{ site.baseurl }}/getting-started/build-tools/#tooling-setup).**
+Download everything: source Sass, JavaScript, and documentation files. **Requires a Sass compiler, [Autoprefixer](https://github.com/postcss/autoprefixer), [postcss-flexbugs-fixes](https://github.com/luisrudge/postcss-flexbugs-fixes), and [some setup]({{ site.baseurl }}/getting-started/build-tools/#tooling-setup).**
 
 <a href="{{ site.download.source }}" class="btn btn-bs btn-outline" onclick="ga('send', 'event', 'Getting started', 'Download', 'Download source');">Download source</a>
 {% endmarkdown %}
@@ -30,7 +30,7 @@ Download everything: source Sass, JavaScript, and documentation files. **Require
 
 ## Package managers
 
-Pull in Bootstrap's **source files** into nearly any project with some of the most popular package managers. No matter the package manager, Bootstrap will **require a Sass compiler and [Autoprefixer](https://github.com/postcss/autoprefixer)** for a setup that matches our official compiled versions.
+Pull in Bootstrap's **source files** into nearly any project with some of the most popular package managers. No matter the package manager, Bootstrap will **require a Sass compiler, [Autoprefixer](https://github.com/postcss/autoprefixer), and [postcss-flexbugs-fixes](https://github.com/luisrudge/postcss-flexbugs-fixes)** for a setup that matches our official compiled versions.
 
 {% callout warning %}
 **Heads up!** Not all package managers have the v4 alpha published yet, but we should have them up shortly!

docs/getting-started/flexbox.md

@@ -19,7 +19,7 @@ Flexbox support is available for a number of Bootstrap's components:
 - 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.
+Vendor prefixes are provided in our compiled CSS with [Autoprefixer](https://github.com/postcss/autoprefixer) via Grunt. Some bugs in IE10-11's Flexbox implementation are worked around via [postcss-flexbugs-fixes](https://github.com/luisrudge/postcss-flexbugs-fixes).
 
 ## Why flexbox?