feat(extensions): most of the extensions implementation, by kdb424

boards/boardsource/4x12/README.md

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+# Boardsource 4x12 Ortho
+
+![Boardsource4x12](https://boardsource.imgix.net/164c3388-5057-46c8-8fcd-82c58c7870ce.jpg?raw=true)
+![Boardsource4x12LP](https://boardsource.imgix.net/c2108ea4-7d70-4327-b4b4-88c8191b1369.jpg?raw=true)
+
+The 4x12 ortholinear keyboard is an extremely common and beloved layout within the keyboard community. Made popular by Jack's Planck from OLKB, the 4x12 ortholinear layout is possibly the most popular non-stagger layout available.  
+
+kb.py is designed to work with the nice!nano  
+
+Retailers (USA)  
+4x12  
+[Boardsource](https://boardsource.xyz/store/5ecb78d286879c9a0c22dafd )  
+Low profile 4x12  
+[Boardsource](https://boardsource.xyz/store/5ecb7dad86879c9a0c22db32)  
+
+Extentions enabled by default  
+- [Layers](https://github.com/KMKfw/kmk_firmware/tree/master/docs/layers.md) Need more keys than switches? Use layers.
+- [MediaKeys](https://github.com/KMKfw/kmk_firmware/tree/master/docs/media_keys.md) Control volume and other media functions
+
+Common Extentions
+- [Power](https://github.com/KMKfw/kmk_firmware/tree/master/docs/power.md) Powersaving features for battery life