Docs: Add browser extension documentation

Fix #2780
Close #2837

packages/hint/docs/user-guide/extensions/extension-browser.md

@@ -0,0 +1,28 @@
+# webhint browser extension
+
+The webhint browser extension surfaces webhint's best practices and hints
+directly in your browser's DevTools. It provides a visual interface that
+allows you to run and re-run site scans that test against all browsers and
+all hint types or to just a select few.
+
+![The webhint browser extension running against webhint.io.](images/extension-browser-0.png)
+
+## Installation
+
+To install the browser extension, click one of the links below and follow the
+installation instructions.
+
+ > Note: The links below redirect to externally-hosted storefronts.
+
+- [webhint browser extension for Google Chrome](https://chrome.google.com/webstore/detail/webhint/gccemnpihkbgkdmoogenkbkckppadcag)
+- [webhint browser extension for Microsoft Edge (Chromium)](https://microsoftedge.microsoft.com/insider-addons/detail/mlgfbihcfnkaenjpdcngdnhcpkdmcdee)
+- [webhint browser extension for Mozilla Firefox](https://addons.mozilla.org/en-US/firefox/addon/webhint/)
+
+## Usage
+
+Open DevTools in the browser. Once the webhint extension is installed, it will
+appear as a tab within DevTools. From here, you can customize the scan by
+selecting which hint categories, browsers, or resources to include or exclude.
+Click "Start scan" to begin scanning the site.
+
+![Choosing settings for a webhint site scan.](images/extension-browser-1.png)