Add markdown style guide

As we're writing more and more stuff in markdown I took the initiative to write a style guide.

This is not meant to be a definitive guide. I wrote it in the way I like to read and write markdown files. I would love to hear your opinion and critique points so we can agree on a common denominator.

Some further points that are not in the guide (yet):

- Prefer link references instead of inline url? [See *link* doc][link doc] or hit *edit* to view how this post uses that style.
- Limit line length at 80? The advantage is that raw markdown reads much nicer. [See example][line length example].

[link doc]: http://daringfireball.net/projects/markdown/syntax#link
[line length example]: https://raw.github.com/upfrontIO/guides/markdown/styles/markdown.md