How to build this project?

I successfully compiled `tests.elaborate` by adding this just above `upstreamCompileOutput()`
```scala
def finalMainClass () = "tests.elaborate.Main"
```

But for `mfccompile` I'm stuck at this part
```bash
> mill tests.mfccompile.compile
[8/265] foreign-modules.dependencies.firrtl.build.firrtlCrossModule.sources.overridden.mill.scalalib.SbtM
[53/265] foreign-modules.dependencies.chisel3.build.chisel3CrossModule.macros.sources.overridden.mill.sca
[85/265] foreign-modules.dependencies.chisel3.build.chisel3CrossModule.core.sources.overridden.mill.scala
[121/265] foreign-modules.dependencies.chisel3.build.chisel3CrossModule.sources.overridden.mill.scalalib.
[151/265] foreign-modules.dependencies.chisel3.build.chisel3CrossModule.plugin.sources.overridden.mill.sc
[262/265] tests.elaborate.elaborate
java.nio.file.NoSuchFileException: /Users/rexyuan/Sources/tilelink/config.json
at ... ()
at os.Path.getInputStream(Path.scala:491)
at os.read$bytes$.apply(ReadWriteOps.scala:256)
at os.read$.apply(ReadWriteOps.scala:216)
at os.read$.apply(ReadWriteOps.scala:214)
at tests.elaborate.Main$.$anonfun$TLCrossBar$1(Main.scala:31)
at ... ()
at ... (Stack trace trimmed to user code only. Rerun with --full-stacktrace to see the full stack trace)
```

What is this `config.json` file and what should I put in it?