Add MD guidelines to Contributing Guide

[barbaricyawps]

In a Slack convo, @bwklein recommends:

Hard-wrapped Markdown is my least favorite kind of Markdown. One sentence per line, and soft-wrap in your editor if you need to.

That sounds like a pretty reasonable approach and an easy rule of thumb to apply. We should consider adding it to our Contributor Guide or an MD guide (if we feel we need a separate guide).

[bwklein]

Some notes:
https://vanemden.wordpress.com/2009/01/01/ventilated-prose/
https://rhodesmill.org/brandon/2012/one-sentence-per-line/
https://asciidoctor.org/docs/asciidoc-recommended-practices/#one-sentence-per-line

Most text editors have soft wrap capabilities now, so an 80 char limits is less useful and can make using ventilated prose and version control unnecessarily complex. (lots of reflowing paragraphs to make it fit the mold.)
Ventilated prose is also great for version control diffs and translations, where sentences are their own 'chunk' based on line breaks.

Comment from Ian: https://thegooddocs.slack.com/team/U01AYHSAAMV
As a final note, we use linters in the following way and this works well for us:

• Vale for language style linting (with a bunch of custom rules)
• markdownlint for linting only Markdown style

https://github.com/DavidAnson/markdownlint-cli2 allows for custom rules.