lld is developed as part of the LLVM project.
See the Creating a Reader guide.
You can run lld with -mllvm -debug command line options to enable debugging printouts. If you want to enable debug information for some specific pass, you can run it with -mllvm '-debug-only=<pass>', where pass is a name used in the DEBUG_WITH_TYPE() macro.
The project documentation is written in reStructuredText and generated using the Sphinx documentation generator. For more information on writing documentation for the project, see the Sphinx Introduction for LLVM Developers.