A new version management plan for dubbo.

[chickenlj]

Hi, all

We have made new a release plan for dubbo. You may have noticed that we have just released 2.6.0 while 2.5.9 is still on progress. From then on, there will be two branches evolving at the same time:

• BugFix Version：lower version，e.g. 2.5.x. This is called the GA version, which can be applied in production. We are supposed only to fix bugs in this version, and increase the third version number when release.
• Feature Version：higher version, e.g. 2.6.x. We add new features to this version, so applications have opportunities try new features. And also we will merge changes made in the GA version.

More details can be find here. We are looking forward to your suggestions.