[docs] Added branch management scheme (resolve #1311)
[docs] Added branch management scheme (resolve #1311)

Introduced beta branch to track the weekly beta version release.
Also updated the branching management rules.

Next will try to automate the beta release process.

acton393/incubator-weex,Hanks10100/incubator-weex,acton393/incubator-weex,acton393/incubator-weex,miomin/incubator-weex,KalicyZhou/incubator-weex,alibaba/weex,Hanks10100/incubator-weex,acton393/incubator-weex,KalicyZhou/incubator-weex,alibaba/weex,Hanks10100/incubator-weex,acton393/incubator-weex,KalicyZhou/incubator-weex,Hanks10100/incubator-weex,acton393/incubator-weex,alibaba/weex,miomin/incubator-weex,miomin/incubator-weex,miomin/incubator-weex,alibaba/weex,acton393/incubator-weex,miomin/incubator-weex,miomin/incubator-weex,miomin/incubator-weex,alibaba/weex,Hanks10100/incubator-weex,alibaba/weex,KalicyZhou/incubator-weex,acton393/incubator-weex,Hanks10100/incubator-weex,alibaba/weex,miomin/incubator-weex,KalicyZhou/incubator-weex,KalicyZhou/incubator-weex,Hanks10100/incubator-weex,Hanks10100/incubator-weex