Documenting Docker release process

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

project/RELEASE-PROCESS.md

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+# Docker Release Process
+
+This document describes how the Docker project is released. The Docker project
+release process targets the Engine, Compose, Kitematic, Machine, Swarm,
+Distribution, Notary and their underlying dependencies (libnetwork, libkv,
+etc...).
+
+Step-by-step technical details of the process are described in 
+[RELEASE-CHECKLIST.md](https://github.com/docker/docker/blob/master/project/RELEASE-CHECKLIST.md).
+
+## Release cycle
+
+The Docker project follows a **time-based release cycle** and ships every nine
+weeks. A release cycle starts the same day the previous release cycle ends.
+
+The first six weeks of the cycle are dedicated to development and review. During
+this phase, new features and bugfixes submitted to any of the projects are
+**eligible** to be shipped as part of the next release. No changeset submitted
+during this period is however guaranteed to be merged for the current release
+cycle.
+
+## The freeze period
+
+Six weeks after the beginning of the cycle, the codebase is officially frozen
+and the codebase reaches a state close to the final release. A Release Candidate
+(RC) gets created at the same time. The freeze period is used to find bugs and
+get feedback on the state of the RC before the release.
+
+During this freeze period, while the `master` branch will continue its normal
+development cycle, no new features are accepted into the RC. As bugs are fixed
+in `master` the release owner will selectively 'cherry-pick' critical ones to
+be included into the RC. As the RC changes, new ones are made available for the
+community to test and review.
+
+This period lasts for three weeks.
+
+## How to maximize chances of being merged before the freeze date?
+
+First of all, there is never a guarantee that a specific changeset is going to
+be merged. However there are different actions to follow to maximize the chances
+for a changeset to be merged:
+
+- The team gives priority to review the PRs aligned with the Roadmap (usually
+defined by a ROADMAP.md file at the root of the repository).
+- The earlier a PR is opened, the more time the maintainers have to review. For
+example, if a PR is opened the day before the freeze date, it’s very unlikely
+that it will be merged for the release.
+- Constant communication with the maintainers (mailing-list, IRC, Github issues,
+etc.) allows to get early feedback on the design before getting into the
+implementation, which usually reduces the time needed to discuss a changeset.
+- If the code is commented, fully tested and by extension follows every single
+rules defined by the [CONTRIBUTING guide](
+https://github.com/docker/docker/blob/master/CONTRIBUTING.md), this will help
+the maintainers by speeding up the review.
+
+## The release
+
+At the end of the freeze (nine weeks after the start of the cycle), all the
+projects are released together.
+
+```
+                                        Codebase              Release
+Start of                                is frozen             (end of the
+the Cycle                               (7th week)            9th week)
++---------------------------------------+---------------------+
+|                                       |                     |
+|           Development phase           |    Freeze phase     |
+|                                       |                     |
++---------------------------------------+---------------------+
+                   6 weeks                      3 weeks
+<---------------------------------------><-------------------->
+```
+
+## Exceptions
+
+If a critical issue is found at the end of the freeze period and more time is
+needed to address it, the release will be pushed back. When a release gets
+pushed back, the next release cycle gets delayed as well.