add svd2funreg readme

.gitignore

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+.*.sw*
+svd2funreg/main

svd2funreg/README.md

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+# svd2funreg
+
+This utility parses an SVD file and produces its funreg implementation.
+
+Compile using `make`, though be sure to fetch the `pugixml` submodule first.
+
+## Usage
+
+```
+./main STM32L476.svd > stm32l476.h
+```
+
+Replace the SVD and H files with those of your choosing.
+