How to build this project?

[RexYuan]

I successfully compiled tests.elaborate by adding this just above upstreamCompileOutput()

def finalMainClass () = "tests.elaborate.Main"

But for mfccompile I'm stuck at this part

> 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?