docs: add userguide file for the central doc
docs: add userguide file for the central doc

See https://wiki.opnfv.org/documentation/userguide .

Change-Id: Id3786ca6d69b517e9dc8ddc96c9cf3a8cf3438ea
Signed-off-by: Ryota MIBU <2a020678b95ddf14a5db916807d3fb3b65997655@cq.jp.nec.com>

opnfv/functest,mywulin/functest,mywulin/functest,opnfv/functest