Be more explicit about future changes to mode

Author: ribasushi

UNIXFS.md

@@ -85,7 +85,7 @@ UnixFS currently supports two optional metadata fields:
   If unspecified this defaults to `0755` for directories/HAMT shards and `0644` for all other types where applicable
   The nine least significant bits represent  `ugo-rwx`
   The next three least significant bits represent `setuid`, `setgid` and the `sticky bit`
-  All others are reserved for future use
+  All others are reserved for future use, and are subject to change - consumers **MUST** mask bits they do not expect, currently `& 07777`
 * `mtime` -- The modification time in seconds since the epoch. This defaults to the unix epoch if unspecified
 
 ### Deduplication and inlining