[Beignet] Versioning and branching policy for beignet project

[Zhigang Gong]

Hi folks,

After some discussion, we made a decision of the versioning and branching policy for
beignet according to the following simple rule.

The master branch always use major . minor version, for example 0.9. And all the new
features and big changes will come to master branch which is for next major relase.
And when everything is ok, we will bump to version 1.0 on master branch.

Everytime when we make a major release, for example the version 0.9, we will create
an new branch to track the following fix version for that release. For 0.9, the branch
name is Release_v0.9.x. We will maintain a series of fix releases that major release.
Only fix patches will be merged into that branch.

In general, the Release_v0.9.x could be treated as the most stable branch before we
move to next major release. On the other hand, the master branch will also include
all the fixes and some bleeding edge new features may not as stable as the previous
major release version.