Merge pull request #702 from jstarks/file_types_where_supported

layer: clarify that not all file types need be supported

Author: jonboulle

layer.md

@@ -30,7 +30,7 @@ Removals are represented using "[whiteout](#whiteouts)" file entries (See [Repre
 
 ### File Types
 
-Throughout this document section, the use of word "files" or "entries" includes:
+Throughout this document section, the use of word "files" or "entries" includes the following, where supported:
 
 * regular files
 * directories