docs(contributing.md): change styleguide title
style(contributing.md): remove backtick syntax

This works only with github-flavored markdown.
style(contributing.md): add comma
docs(contributing.md): import more from docs
docs(contributing): lower commit message length

See https://github.com/blog/926-shiny-new-commit-styles on the reason
behind this change. A commit message body longer than 72 characters is
wrapped.
docs(contributing): change "refactor" to "ref"
Merge pull request #1437 from deis/commit-guideline

ref(contributing): changes to the commit style guideline