Add comment with link to file permissions documentation

[skip ci]

Author: thekid

src/main/php/xp/lambda/PackageLambda.class.php

@@ -8,6 +8,7 @@
 class PackageLambda {
   const COMPRESSION_THRESHOLD= 24;
 
+  // See https://www.php.net/manual/en/function.fileperms.php
   const IS_LINK= 0120000;
   const IS_FILE= 0100000;
   const IS_FOLDER= 0040000;