Versioning scheme

gfx-rs adheres to Rust-flavored semantic versioning (SemVer). Releases are bound to breaking version changes of gfx-hal, since roads of gfx-rs lead to it. Here is a rough schedule:

• development is done in master, including the breaking changes
• eventually, a PR includes a change to Cargo.toml with a new breaking version
• naturally, it bumps the dependencies' (most importantly, gfx-backend-xxx crates) versions
• PR is reviewed and merged; it is required to carry the corresponding CHANGELOG.md notes
• we publish all the crates
• we create a hal-0.x tag, and an associated branch
• we announce the release to the community
• any patches to this version go to the associated branch, from which the patch versions are published