readme: convert to AsciiDoc

[hmdne]

This fixes #122

Metanorma PR checklist

• [ ] Breaking changes (list related PRs)
• [ ] Documentation update required (create task for this)
• [ ] External dependency introduced (documentation update need)
• [ ] Gem with native library introduced

[ronaldtse]

Actually we want to combine the READMEs into one. There's insufficient information on how to use Coradoc and build documents with it.