Add preferred style guide to CONTRIBUTING.

Sometimes there are multiple possible behaviors that are all valid markdown. E.g.: https://github.com/plasticboy/vim-markdown/pull/70#issuecomment-39181974

We should therefore fix one preferred style.

With all modesty, the best styleguide I could find so far was the one I wrote.

In particular:

- it is compatible with the current behavior of Vim markdown
- one of its design goals is to be easy to implement in text editors: http://www.cirosantilli.com/markdown-styleguide/#design-goals
- I control it, and I will give high priority to suggestions coming from Vim Markdown collaborators
- it is [used by GitLab](https://github.com/gitlabhq/gitlabhq/blob/master/CONTRIBUTING.md#style-guides), which has a measly 13k stars =)

If anyone finds a better option, or wishes to modify one of its points, I'd love to hear it.