Longhorn version and repository branching tagging policy

1. master branch is always stable. It would only merge the changes that considered stable and passed integration test. It can have tags e.g. v0.3.0 or v0.3.1 when v0.3.x is the latest stable release.
2. The branches like v0.2 are for the previous stable releases. It can have tags like v0.2.0 and v0.2.1 which contains the previous minor stable releases. This branch will be created from the master immediately before the next rc branch was merged in.
3. The branches like v0.3-rc are for the release candidates. It's where the development happening immediately before the release. The branch will be merged into master using merge (instead of rebase) before the official release. It can have tags like v0.3-rc3 for the release candidates. The branch will be deleted after the release.