[xzwxflda] Spaces matter in Markdown

Enumerate the situations in Markdown in which whitespace matters.  Inspired by this gotcha with unordered lists.

* one
* two
* three
* four

versus

*one
*two
*three
*four

The latter gets rendered as a paragraph of alternating italicized and non-italicized text: one two three four.

Previously, on other special characters which may trip you up in Markdown.