docs: add story about autoprefixer config

Those wishing to change which browsers are targeted for when css is prefixed can do so with a simple addition to their package.json file

Close #5975

Author: markgoho

docs/documentation/stories/autoprefixer.md

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+# Change target browsers for Autoprefixer
+
+Currently, the CLI uses [Autoprefixer](https://github.com/postcss/autoprefixer) to ensure compatibility
+with different browser and browser versions. You may find it necessary to target specific browsers
+or exclude certain browser versions from your build.
+
+Internally, Autoprefixer relies on a library called [Browserslist](https://github.com/ai/browserslist)
+to figure out which browsers to support with prefixing.
+
+There are a few ways to tell Autoprefixer what browsers to target:
+
+### Add a browserslist property to the `package.json` file
+```
+"browserslist": [
+  "> 1%",
+  "last 2 versions"
+]
+```
+
+### Add a new file to the project directory called `.browserslistrc`
+```
+### Supported Browsers
+
+> 1%
+last 2 versions
+```
+
+Autoprefixer will look for the configuration file/property to use when it prefixes your css.
+Check out the [browserslist repo](https://github.com/ai/browserslist) for more examples of how to target
+specific browsers and versions.
+
+_Side note:_
+Those who are seeking to produce a [progressive web app](https://developers.google.com/web/progressive-web-apps/) and are using [Lighthouse](https://developers.google.com/web/tools/lighthouse/) to grade the project will
+need to add the following browserslist config to their package.json file to eliminate the [old flexbox](https://developers.google.com/web/tools/lighthouse/audits/old-flexbox) prefixes:
+
+`package.json` config:
+```
+"browserslist": [
+  "last 2 versions",
+  "not ie <= 10",
+  "not ie_mob <= 10"
+]
+```