Document branches and tags

Signed-off-by: Arnaud Porterie <arnaud.porterie@docker.com>

project/BRANCHES-AND-TAGS.md

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+Branches and tags
+=================
+
+Note: details of the release process for the Engine are documented in the
+[RELEASE-CHECKLIST](https://github.com/docker/docker/blob/master/project/RELEASE-CHECKLIST.md).
+
+# Branches
+
+The docker/docker repository should normally have only three living branches at all time, including
+the regular `master` branch:
+
+## `docs` branch
+
+The `docs` branch supports documentation updates between product releases. This branch allow us to
+decouple documentation releases from product releases.
+
+## `release` branch
+
+The `release` branch contains the last _released_ version of the code for the project.
+
+The `release` branch is only updated at each public release of the project. The mechanism for this
+is that the release is materialized by a pull request against the `release` branch which lives for
+the duration of the code freeze period. When this pull request is merged, the `release` branch gets
+updated, and its new state is tagged accordingly.
+
+# Tags
+
+Any public release of a compiled binary, with the logical exception of nightly builds, should have
+a corresponding tag in the repository.
+
+The general format of a tag is `vX.Y.Z[-suffix[N]]`:
+
+- All of `X`, `Y`, `Z` must be specified (example: `v1.0.0`)
+- First release candidate for version `1.8.0` should be tagged `v1.8.0-rc1`
+- Second alpha release of a product should be tagged `v1.0.0-alpha1`