[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.

Thanks! I added some additional stylistic comments. They are all optional though, so apply what makes sense to you.

About the words "DWARF", "CodeView", and "PDB", I wasn't sure whether to treat them as "code" (and put in monospace) or just leave them alone without styling... Anyway, probably best to choose one approach for such words and use it consistently throughout.

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