[llvm] [llvm-debuginfo-analyzer][DOC] Convert 'README.txt' to markdown. (PR #86394)

[J. Ryan Stinnett via llvm-commits]

https://github.com/jryans approved this pull request.

Looks good overall, thanks! 😄 I noted a few optional stylistic points for consideration.

https://github.com/llvm/llvm-project/pull/86394