samples/zoap-server: Update docs with information about libcoap

Add information about how to run libcoap's[1] implementation of the
ETSI testcases against zoap-server.

Jira: ZEP-1538

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

 (branch develop)

Change-Id: I6aa5989575cc15630371aa0761bee98fb6fe1b80
Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>