Update the readme

README.md

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 # codegen
 
+A script that turns `[filename].gen.ts` files into `[filename].ts` files by running the function exported by the `.gen.ts` files and putting the result of that function into the output `.ts` file. This is useful for any kind of codegen such as if you want to have a file that wants to do bulk imports of all the ts files in a different directory, or codegen database schema types, and so on.
+
 Use:
 
 ```sh