README.md: included cross-reference to libcoap-minimal
README.md: included cross-reference to libcoap-minimal

libcoap-minimal [1] contains small client and server examples that
illustrate the use of libcoap-2.

This change also updates the copyright year.

[1] https://github.com/obgm/libcoap-minimal

authmillenon/libcoap,authmillenon/libcoap,authmillenon/libcoap