Add a note about padded vocab size during training

README.md

@@ -193,6 +193,8 @@ The BLOOM tokenizer ([link](https://huggingface.co/bigscience/tokenizer)) is a l
 
 - A vocabulary size of 250,680
 
+The vocabulary size was padded to 250,880 for practical purposes during training, but the effective model vocabulary size is 250,680.
+
 It was trained on a subset of a preliminary version of the corpus using alpha-weighting per language.    
     
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