doc(add): document hash stability

fixes #4318

core/commands/add.go

@@ -102,6 +102,11 @@ You can now check what blocks have been created by:
   QmY6yj1GsermExDXoosVE3aSPxdMNYr6aKuw3nA8LoWPRS 2059
   QmerURi9k4XzKCaaPbsK6BL5pMEjF7PGphjDvkkjDtsVf3 868
   QmQB28iwSriSUSMqG2nXDTLtdPHgWb4rebBrU7Q1j4vxPv 338
+
+Finally, a note on hash determinism. While not guaranteed, adding the same
+file/directory with the same flags will almost always result in the same output
+hash. However, almost all of the flags provided by this command (other than pin,
+only-hash, and progress/status related flags) will change the final hash.
 `,
 	},