docs: add Jenkins CI Badge
docs: add Jenkins CI Badge

https://jenkins-rkt-public.prod.coreos.systems/

intelsdi-x/rkt,squeed/rkt,jonboulle/rkt,rkt/rkt,joshix/rkt,monstermunchkin/rkt,Wilybald/rkt,kinvolk/rkt,squeed/rkt,sgotti/rkt,jjlakis/rkt,akshaykarle/rkt,dgonyeo/rkt,coreos/rkt,lucasallan/rkt,euank/rkt,achanda/rkt,monstermunchkin/rkt,sstabellini/rkt,akshaykarle/rkt,alban/rkt,sstabellini/rkt,Wilybald/rkt,euank/rkt,coreos/rkt,jakon89/rkt,ybubnov/rkt,shobhit85/rkt,monstermunchkin/rkt,yifan-gu/rocket,intelsdi-x/rkt,yifan-gu/rkt,sgotti/rkt,cab105/rkt,sgotti/rkt,akshaykarle/rkt,lucasallan/rkt,cab105/rkt,nhlfr/rkt,euank/rkt,kinvolk/rkt,Shashwatsh/rkt,achanda/rkt,s-urbaniak/rkt,Wilybald/rkt,yifan-gu/rocket,coreos/rkt,lucasallan/rkt,rkt/rkt,shobhit85/rkt,nhlfr/rkt,squeed/rkt,kinvolk/rkt,shobhit85/rkt,achanda/rkt,dgonyeo/rkt,sstabellini/rkt,joshix/rkt,rkt/rkt,dgonyeo/rkt,Shashwatsh/rkt,Wilybald/rkt,s-urbaniak/rkt,ybubnov/rkt,jonboulle/rkt,alban/rkt,yifan-gu/rocket,jakon89/rkt,alban/rkt,jakon89/rkt,s-urbaniak/rkt,Shashwatsh/rkt,intelsdi-x/rkt,joshix/rkt,nhlfr/rkt,jonboulle/rkt,yifan-gu/rkt,cab105/rkt,kinvolk/rkt,lucasallan/rkt,rkt/rkt,joshix/rkt,yifan-gu/rkt,ybubnov/rkt,jjlakis/rkt,jjlakis/rkt