Update README.md
Update README.md

Slightly expanded description

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 # cmoy-47-variant-amp
-Headphone amp for the NE5532 that is available on eBay and other marketplaces in the late 2010s to present 2022.
+Headphone amp for the NE5532 that is available on eBay and other marketplaces in the late 2010s to present 2022. PCB reverse-engineered on April 2022.