Add req to section "ONAP Heat VNF Modularity"

.. req::
:id: R-610015
:target: VNF
:keyword: MAY
:validation_mode: none
:introduced: guilin

When a VNF's Heat Orchestration Template is composed of a Base Module and
one or more Incremental Modules, the Base Module *SHOULD NOT* declare an
``OS::Nova::Server`` resource.
When a VNF is composed of a Base Module and one or more Incremental
Modules, the ONAP VF Module Replacement Feature does not support
the replacement (updating) of the Base Module.
An ``OS::Nova::Server`` resource declared in the Base Module can not be
updated.