Document linguist overriding

Users report that some languages aren't detected, or at least not
reported. This is mostly due to the fact that these languages aren't
programming languages, according to Linguist.

Originally noted in: https://gitlab.com/gitlab-org/gitlab-ce/issues/50705#note_99600216

Related: https://gitlab.com/gitlab-org/gitlab-ce/issues/51995

Author: ZJvandeWeg

doc/user/project/repository/index.md

@@ -166,7 +166,11 @@ minutes.
 ![Repository Languages bar](img/repository_languages.png)
 
 Not all files are detected, among others; documentation, 
-vendored code, and most markup languages are excluded.
+vendored code, and most markup languages are excluded. This behaviour can be
+adjusted by overriding the default. For example, to enable `.proto` files to be
+detected, add the following to `.gitattributes` in the root of your repository.
+
+> *.proto linguist-detectable=true
 
 ## Compare