Add docs about inspecting preprocessor/plugin output

[brandonchinn178]

Add a note in the README with flags users can use to inspect the output of this tool, namely:

1. To inspect the generated Main module, build with -keep-tmp-files, and look in $TMPDIR for a .hspp file
   • ref: https://gitlab.haskell.org/ghc/ghc/-/issues/22258
   • ref: commercialhaskell/stack#5892
2. To inspect the converted test modules, build with -ddump-rn -ddump-to-file and look for .dump-rn file in .stack-work/