add OWNERS for client-go

staging/src/k8s.io/client-go/OWNERS

+approvers:
+- caesarxuchao
+- deads2k
+- krousey
+- lavalamp
+- smarterclayton
+reviewers:
+- thockin
+- lavalamp
+- smarterclayton
+- wojtek-t
+- deads2k
+- yujuhong
+- derekwaynecarr
+- caesarxuchao
+- vishh
+- mikedanese
+- liggitt
+- nikhiljindal
+- bprashanth
+- gmarek
+- erictune
+- davidopp
+- pmorie
+- sttts
+- kargakis
+- dchen1107
+- saad-ali
+- zmerlynn
+- luxas
+- janetkuo
+- justinsb
+- roberthbailey
+- ncdc
+- timstclair
+- yifan-gu
+- eparis
+- mwielgus
+- timothysc
+- feiskyer
+- jlowdermilk
+- soltysh
+- piosz
+- jsafrane