Documentation/devel/release.md: small updates
Documentation/devel/release.md: small updates

Small updates explaining how to do a release:
- include the "fly" stage1
- nicer "sed" command to get the authors
- use more recent version as example
- suggest to add a vUNRELEASED section in CHANGELOG.md

achanda/rkt,kinvolk/rkt,blixtra/rkt,il9ue/rkt,kbrwn/rkt,coreos/rkt,yifan-gu/rocket,markllama/rkt,linki/rkt,alban/rkt,squeed/rkt,akshaykarle/rkt,cab105/rkt,s-urbaniak/rkt,kinvolk/rkt,yifan-gu/rocket,il9ue/rkt,jonboulle/rkt,monstermunchkin/rkt,joshix/rkt,nhlfr/rkt,akshaykarle/rkt,yifan-gu/rkt,sstabellini/rkt,lucasallan/rkt,jonboulle/rkt,intelsdi-x/rkt,dgonyeo/rkt,yifan-gu/rocket,cab105/rkt,markllama/rkt,nhlfr/rkt,matthaias/rkt,sinfomicien/rkt,shobhit85/rkt,il9ue/rkt,rkt/rkt,nhlfr/rkt,euank/rkt,Shashwatsh/rkt,kbrwn/rkt,dgonyeo/rkt,mike1729/rkt,jakon89/rkt,lucasallan/rkt,shobhit85/rkt,markllama/rkt,monstermunchkin/rkt,rkt/rkt,Shashwatsh/rkt,sgotti/rkt,sgotti/rkt,alban/rkt,endocode/rkt,sstabellini/rkt,tmrts/rkt,coreos/rkt,blixtra/rkt,Wilybald/rkt,cab105/rkt,ybubnov/rkt,Shashwatsh/rkt,joshix/rkt,Wilybald/rkt,s-urbaniak/rkt,linki/rkt,jonboulle/rkt,endocode/rkt,sgotti/rkt,euank/rkt,dgonyeo/rkt,mike1729/rkt,ybubnov/rkt,rkt/rkt,Wilybald/rkt,endocode/rkt,Wilybald/rkt,jjlakis/rkt,tmrts/rkt,achanda/rkt,matthaias/rkt,jakon89/rkt,yifan-gu/rkt,kbrwn/rkt,mike1729/rkt,alban/rkt,blixtra/rkt,s-urbaniak/rkt,kinvolk/rkt,intelsdi-x/rkt,jjlakis/rkt,sstabellini/rkt,joshix/rkt,jakon89/rkt,kinvolk/rkt,euank/rkt,matthaias/rkt,monstermunchkin/rkt,sinfomicien/rkt,akshaykarle/rkt,achanda/rkt,squeed/rkt,rkt/rkt,lucasallan/rkt,lucasallan/rkt,sinfomicien/rkt,ybubnov/rkt,joshix/rkt,shobhit85/rkt,linki/rkt,tmrts/rkt,squeed/rkt,jjlakis/rkt,yifan-gu/rkt,intelsdi-x/rkt,coreos/rkt