Add man page

[werdahias]

I am in the process of packaging your crate for debian. /usr/bin/qrencode has no man page. I started writing the man page, but I have no idea what the options etc. are. I attached it below.
qrencode.md

[fc7]

I think the binary is only shipped as a minimalistic/simplistic demo, and is not really intended for real-world usage. It only takes a single argument (the QrCode content) and prints it as a simple string to STDOUT. A more useful version would offer the possibility to control all rendering options (image format, svg) and influence the error correction level, etc.