Convert `README.md` to `README.adoc`

Asciidoctor offers more features than Markdown which this README can
benefit from (attributes, table of contents, description lists, and
non-HTML internal anchors to name a few).

On GitHub, the table of contents is placed right after the preamble,
before the first section's heading. Otherwise, it's placed on the left
for increased readability (not supported by GitHub).

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Id4965abaa7151cb0fc3caca290fcf5d3669f0b80