Update README.md

Author: lessw2020

README.md

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 Ranger - a synergistic optimizer combining RAdam (Rectified Adam) and LookAhead, and now GC (gradient centralization) in one optimizer.
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+## quick note - Ranger21 is now in beta and is Ranger with a host of new improvements.  Recommend you compare results with Ranger21:  https://github.com/lessw2020/Ranger21
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 ### Latest version 20.9.4 - updates Gradient Centralization to GC2 (thanks to GC developer) and removes addcmul_ deprecation warnings in PyTorch 1.60. 
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 *Latest version is in ranger2020.py - looking at a few other additions before integrating into the main ranger.py.