MODEL_REVISION & TOKENIZER_REVISION: Both are needed to configure the revision

[TimPietrusky]

When someone wants to use a different revision of a model, they need to specify the revision. Looking at the README, it is not clear how to do that. My first assumption would be to use MODEL_REVISION, but that is not enough (and also not documented in the env-variables section).
When you use the TOKENIZER_REVISION (which is documented), then it is also not working.
Only when using both the MODEL_REVISION and the TOKENIZER_REVISION, then it is working.

I think it would make sense to document this in the README, so that users are sure what is happening.

@pandyamarut what do you think about this?