Update RELEASE_NOTES.md

Author: DanielGoldfarb

RELEASE_NOTES.md

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+### <a name="v0.12.7aN"></a>v0.12.7aN &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; TO BE released 2021-MM-DD
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+- <a name="v0.12.7a8"></a>v0.12.7a8 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; merged 2021-02-23
+  - support `yscale` kwarg.  (See also [**`yscale.ipynb`**](https://github.com/matplotlib/mplfinance/blob/yscaleIss21/examples/yscale.ipynb), and [issue 21](https://github.com/matplotlib/mplfinance/issues/21)).
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+---
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 ### <a name="v0.12.7a7"></a>v0.12.7a7 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; released 2021-02-22
 
 - Support [**`type=hollow_and_filled` candles**](https://github.com/matplotlib/mplfinance/blob/master/examples/hollow_and_filled_candles.ipynb). &nbsp; (Thank you [Kenan Arik](https://github.com/KenanHArik))