Improve README.md

The README.md file has been converted to Markdown and been slightly extended.

Here are some further changes I'd like to see:

Not sure if it's still portable to Hugs. We claim it is, but I doubt it.

The section about "Using binary". I think this section is not too helpful. It could be more structured of the alternatives and with links to the relevant haddocks.

Deriving binary instance section. This is outdated and missleading. We can now generate instances with GHCs generics. A small example of how to do so, or link to the haddocks, would be better suiting.

Empieza leyendo README.md, centrándote en la afirmación de portabilidad de Hugs, la sección “Using binary” y la sección sobre deriving. Comprueba los Haddocks actuales y las indicaciones sobre GHC generics antes de editar; se considera terminado cuando README describe con precisión la portabilidad, presenta alternativas de uso con enlaces relevantes y muestra o enlaza al enfoque moderno para generar instancias.

Escrito por el modelo de indexación a partir del texto del issue.