Document AsciiDoc guidelines

The development documentation contains information on our code style. It does not explain our AsciiDoc style guidelines. See also:

• #54 (closed) for one sentence per line etc
• #236 (closed) for book/paper references etc
• #257 (closed) where we configured some existing Checkstyle checks for AsciiDoc files
• #271 where we ideas for more Checkstyle checks
• ...

We could also document how we work with AsciiDoc:

• We support various output formats
• The structure of the files, images, _local_attributes etc files
• ...