doc: add RCU integration design details
doc: add RCU integration design details

Add a section to describe a design to integrate QSBR RCU library
with other libraries in DPDK.

Signed-off-by: Honnappa Nagarahalli <01ccf438b971cc1d8919c8a7f18443cae9429d8b@arm.com>
Signed-off-by: Ruifeng Wang <a548d51fbc42485a8ba807711a537e88c2b59dd2@arm.com>
Reviewed-by: Gavin Hu <207ee22c2cf71e20c5b5df8395050c07c07d8709@arm.com>
Reviewed-by: Ruifeng Wang <a548d51fbc42485a8ba807711a537e88c2b59dd2@arm.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk