first draft of docs

doc/user/project/highlighting.md

+[Rouge]: https://rubygems.org/gems/rouge
+
+# Syntax Highlighting
+
+GitLab provides syntax highlighting on all files and snippets through the [Rouge][] rubygem. It will try to guess what language to use based on the file extension, which most of the time is sufficient.
+
+If GitLab is guessing wrong, you can override its choice of language using the `gitlab-language` attribute in `.gitattributes`. For example, if you are working in a Prolog project and using the `.pl` file extension (which would normally be highlighted as Perl), you can add the following to your `.gitattributes` file:
+
+``` conf
+*.pl gitlab-language=prolog
+```
+
+When you check in and push that change, all `*.pl` files in your project will be highlighted as Prolog.
+
+The paths here are simply git's builtin [`.gitattributes` interface](https://git-scm.com/docs/gitattributes).  So, if you were to invent a file format called a `Nicefile` at the root of your project that used ruby syntax, all you need is:
+
+``` conf
+/Nicefile gitlab-language=ruby
+```
+
+To disable highlighting entirely, use `gitlab-language=text`. Lots more fun shenanigans are available through CGI options, such as:
+
+```
+# json with erb in it
+/my-cool-file gitlab-language=erb?parent=json
+
+# an entire file of highlighting errors!
+/other-file gitlab-language=text?token=Error
+```