confroller/dpset: Add usage example of instantiation
confroller/dpset: Add usage example of instantiation

This patch adds the example to explain how to register dpset.DPSet
service and how to get DPSet instance from user application in order to
call the API of DPSet.

Signed-off-by: IWASE Yusuke <bb80e84cc9f5a8f01385872ee89032beb5d4eba7@gmail.com>
Signed-off-by: FUJITA Tomonori <93dac1fe9c4b2a3957982200319981492ad4976e@lab.ntt.co.jp>

fujita/ryu,fujita/ryu,fujita/ryu,lagopus/ryu-lagopus-ext,lagopus/ryu-lagopus-ext,fujita/ryu,osrg/ryu,iwaseyusuke/ryu,lagopus/ryu-lagopus-ext,osrg/ryu,iwaseyusuke/ryu,lagopus/ryu-lagopus-ext,iwaseyusuke/ryu,osrg/ryu,osrg/ryu,fujita/ryu,osrg/ryu,iwaseyusuke/ryu,iwaseyusuke/ryu,lagopus/ryu-lagopus-ext