Add file finder documentation

[ci skip]

doc/workflow/README.md

@@ -6,6 +6,7 @@
 - [GitLab Flow](gitlab_flow.md)
 - [Groups](groups.md)
 - [Keyboard shortcuts](shortcuts.md)
+- [File finder](file_finder.md)
 - [Labels](labels.md)
 - [Notification emails](notifications.md)
 - [Project Features](project_features.md)

doc/workflow/file_finder.md

+# File finder
+
+_**Note:** This feature was [introduced][gh-9889] in GitLab 8.4._
+
+---
+
+The file finder feature allows you to quickly shortcut your way when you are
+searching for a file in a repository using the GitLab UI.
+
+You can find the **Find File** button when in the **Files** section of a
+project.
+
+![Find file button](img/file_finder_find_button.png)
+
+---
+
+For the more lazy, there is a [shortcut button](shortcuts.md) as well.
+
+Go the **Files** section of a project and press `t` on your keyboard to launch
+the search function. Start typing what you are searching for and watch the
+magic being unfold. With the up/down arrows, you go up and down the results,
+with `Esc` you close the search and go back to **Files**.
+
+## How it works
+
+The File finder feature is powered by the [Fuzzy filter] library.
+
+It implements a fuzzy search with highlight, and tries to provide intuitive
+results by recognizing patterns that people use while searching.
+
+For example, consider the [GitLab CE repository][ce] and that we want to open
+the `app/controllers/admin/deploy_keys_controller.rb` file.
+
+Using fuzzy search, we start by typing letters that get us closer to the file.
+
+**Protip:** To narrow down your search, include `/` in your search terms.
+
+![Find file button](img/file_finder_find_file.png)
+
+[gh-9889]: https://github.com/gitlabhq/gitlabhq/pull/9889 "File finder pull request"
+[fuzzy filter]: https://github.com/jeancroy/fuzzaldrin-plus "fuzzaldrin-plus on GitHub"
+[ce]: https://gitlab.com/gitlab-org/gitlab-ce/tree/master "GitLab CE repository"