Mention PEER acls in readme

README.md

@@ -197,8 +197,16 @@ format:
 > *identity* *path* *operation*
 
 where operation can be either `read` or `write` (possibly abbreviated
-to `r` or `w`). Pass this file to "replds server" using the *--acls*
-option. Lines that start with a # are considered comments and ignored.
+to `r` or `w`), or:
+
+> *identity* *operation*
+
+to be used with the `peer` operation (that takes no path component), a
+special operation that covers the internal synchronization protocol.
+
+Pass the file containing ACL rules to "replds server" using the
+*--acls* option. Lines that start with a # are considered comments and
+ignored.
 
 ### Triggers