Add BOM note to CQ docs

doc/user/project/merge_requests/code_quality.md

@@ -362,6 +362,8 @@ Example:
 NOTE:
 Although the Code Climate spec supports more properties, those are ignored by
 GitLab.
+The GitLab parser does not allow a [byte order mark](https://en.wikipedia.org/wiki/Byte_order_mark)
+at the beginning of the file.
 
 ## Code Quality reports **(PREMIUM)**