README.md: Small formatting changes

To make the README visually a bit more pleasing, this patch reformats it a little:

 * The complexity explanation was inlined and wrapped in code blocks
 * Homepage and Usage were turned into headers
 * The gcc/g++ command lines were broken up into multiple lines, and wrapped in a shell code block

Signed-off-by: Gergely Nagy <algernon@balabit.hu>