layer: document Windows-specific file attributes

Signed-off-by: John Starks <[email protected]>

Author: jstarks

layer.md

@@ -88,6 +88,16 @@ END FOR
 
 With this approach, the link map and links names of a directory could be compared against that of another directory to derive additions and changes to hardlinks.
 
+#### Platform-specific attributes
+
+Implementations on Windows MUST support these additional attributes, encoded in [PAX vendor
+extensions](https://github.com/libarchive/libarchive/wiki/ManPageTar5#pax-interchange-format) as follows:
+
+* [Windows file attributes](https://msdn.microsoft.com/en-us/library/windows/desktop/gg258117(v=vs.85).aspx) (`MSWINDOWS.fileattr`)
+* [Security descriptor](https://msdn.microsoft.com/en-us/library/cc230366.aspx) (`MSWINDOWS.rawsd`): base64-encoded self-relative binary security descriptor
+* Mount points (`MSWINDOWS.mountpoint`): if present on a directory symbolic link, then the link should be created as a [directory junction](https://en.wikipedia.org/wiki/NTFS_junction_point)
+* Creation time (`LIBARCHIVE.creationtime`)
+
 ## Creating
 
 ### Initial Root Filesystem