WS14-SD: Switch to better syntax for line breaks

Our Markdown engine (Kramdown) supports this alternative syntax [1, 2], which
is better supported by the Git toolchain and is more likely to be
preserved by editors (though I fear it is less standard).

[1] http://kramdown.gettalong.org/quickref.html
[2] http://kramdown.gettalong.org/syntax.html