Update git submodules

Update git submodules

* Update docs/submodules/releng-xci from branch 'master'
  - Merge "Add documentation for user guide and sandbox"
  - Add documentation for user guide and sandbox

    This change adds the user guide and sandbox information.
    The documentation is created by using the information we
    have on Wiki and README.rst.

    Wiki pages and README.rst will be removed once all the
    documentation is moved to docs.opnfv.org.

    Parts of the README.rst such as more advanced ways to use
    the sandbox will be placed in developer guide and it is
    accessible by a link placed in user guide.

    Change-Id: I7214a53d7ab39125d2164c5b3468b1ba18933b31
    Signed-off-by: Fatih Degirmenci <fatih.degirmenci@ericsson.com>