Add man page

[sorairolake]

This is written in AsciiDoc or Pandoc’s Markdown.

These can be converted by:

• asciidoctor -b manpage doc/hexyl.1.adoc
• pandoc -s -f markdown -t man -o doc/hexyl.1 doc/hexyl.1.md

I will fix to add a conversion process to the CI, add a troff file as well, keep only it, or other. Which one is better?

[sharkdp]

Awesome. Thank you very much for working on this. I would prefer the pandoc Markdown => man page way because I am familiar with this from other projects. And I don't know AsciiDoc.

[sorairolake]

@sharkdp OK. Added the conversion process from Pandoc's Markdown in the CI. Also, I wrote the conversion method in the README.