README: add notice for maint branch
README: add notice for maint branch

- modify yobi download guide to specifying maint branch with git
- add warining about branch for install on a production server

Reviewed-by: JiHan Kim <1950223916caa471f669992971896ca29d8b0d4f@navercorp.com>

ahb0327/yobi,doortts/fork-yobi,ihoneymon/yobi,brainagenet/yobi,doortts/fork-yobi,oolso/yobi,brainagenet/yobi,yona-projects/yona,Limseunghwan/oss,bloodybear/yona,Limseunghwan/oss,bloodybear/yona,yona-projects/yona,bloodybear/yona,oolso/yobi,violetag/demo,doortts/fork-yobi,ahb0327/yobi,bloodybear/yona,ihoneymon/yobi,oolso/yobi,naver/yobi,doortts/forked-for-history,ahb0327/yobi,violetag/demo,brainagenet/yobi,doortts/forked-for-history,doortts/forked-for-history,doortts/fork-yobi,yona-projects/yona,naver/yobi,yona-projects/yona,naver/yobi,Limseunghwan/oss,ihoneymon/yobi