odhcpd: convert README to markdown

odhcpd: convert README to markdown

Convert the README file to GitHub style markdown. I don't think it affects the
readability of the plain text file much (or not at all, rather). But it makes
it much easier to read for people looking at the GitHub repo.

I've intentionally kept the fancy markup at a minimum and also haven't changed
any help text messages in order to not get bogged down in how each help message
should read. That is something which can be addressed later.

Signed-off-by: David Härdeman <[email protected]>
Link: https://github.com/openwrt/odhcpd/pull/263
Signed-off-by: Álvaro Fernández Rojas <[email protected]>