Add comment about RNG seed

lib/settings.lua

@@ -18,7 +18,7 @@ cmd:text()
 cmd:text("waifu2x-training")
 cmd:text("Options:")
 cmd:option("-gpu", -1, 'GPU Device ID')
-cmd:option("-seed", 11, 'RNG seed')
+cmd:option("-seed", 11, 'RNG seed (note: it only able to reproduce the training results with `-thread 1`)')
 cmd:option("-data_dir", "./data", 'path to data directory')
 cmd:option("-backend", "cunn", '(cunn|cudnn)')
 cmd:option("-test", "images/miku_small.png", 'path to test image')