Added documentation chapter for Git attributes

As discussed in https://gitlab.com/gitlab-org/gitlab_git/issues/28 we'll
need to clearly document the need for .gitattributes files being encoded
using UTF-8.

[ci skip]

CHANGELOG

@@ -14,6 +14,7 @@ v 8.13.0 (unreleased)
   - Clarify documentation for Runners API (Gennady Trafimenkov)
   - Change user & group landing page routing from /u/:username to /:username
   - Prevent running GfmAutocomplete setup for each diff note !6569
+  - Added documentation for .gitattributes files
   - AbstractReferenceFilter caches project_refs on RequestStore when active
   - Replaced the check sign to arrow in the show build view. !6501
   - Add a /wip slash command to toggle the Work In Progress status of a merge request. !6259 (tbalthazar)

doc/README.md

@@ -20,6 +20,7 @@
 - [Webhooks](web_hooks/web_hooks.md) Let GitLab notify you when new code has been pushed to your project.
 - [Workflow](workflow/README.md) Using GitLab functionality and importing projects from GitHub and SVN.
 - [University](university/README.md) Learn Git and GitLab through videos and courses.
+- [Git Attributes](user/project/git_attributes.md) Managing Git attributes using a `.gitattributes` file.
 
 ## Administrator documentation
 

doc/user/project/git_attributes.md

+# Git Attributes
+
+GitLab supports defining custom [Git attributes][gitattributes] such as what
+files to treat as binary, and what language to use for syntax highlighting
+diffs.
+
+To define these attributes, create a file called `.gitattributes` in the root
+directory of your repository and push it to the default branch of your project.
+
+## Encoding Requirements
+
+The `.gitattributes` file _must_ be encoded in UTF-8 and _must not_ contain a
+Byte Order Mark. If a different encoding is used, the file's contents will be
+ignored.
+
+## Syntax Highlighting
+
+The `.gitattributes` file can be used to define which language to use when
+syntax highlighting files and diffs. See ["Syntax
+Highlighting"](highlighting.md) for more information.
+
+[gitattributes]: https://git-scm.com/docs/gitattributes