added readme for mathgen

core/mathgen/README.md

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+# mathgen, or why I'm so so sorry
+
+Go's type system is a double-edged sword, and the inner edge will definitely get
+you when you go to do generic math. I opted against using the `reflect` library,
+and instead went for codegen. So while the result is barely readable, it will at
+least be somewhat faster, theoretically.
+
+This module generates the `math.go` file which resides in the core library. The
+`mathgen.tpl` file is where most of the magic happens. It is virtually
+unreadable. The biggest reason for this is that I want it to be able to pass
+through `go fmt` unphased, so that it doesn't end up getting changed back and
+forth throughout the code's history.
+
+## Running
+
+You can run code gen by running `make gen` in the `core/` directory. The
+`Makefile` in the root of the project should also reference this, so running
+that in the root should also result in the file getting regenerated.