RELEASES: add formal release policy for runc

Historically, our release cadence and support policy has been quite
ad-hoc, which has caused some strife and is not really becoming of a
project as widely used as ours. So, in an attempt to make our releases
more regular and to provide more guidance on our backport policy, add a
document outlining our policy.

Signed-off-by: Aleksa Sarai <[email protected]>

RELEASES.md

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+## Release Cadence and Support Policy ##
+
+This document describes the release cadence for runc as well as outlining the
+support policy for old release branches. Historically, despite runc being the
+most widely used Linux container runtime, our release schedule has been very
+ad-hoc and has resulted in very long periods of time between minor releases,
+causing issues for downstreams that wanted particular features.
+
+### Semantic Versioning ###
+
+runc uses [Semantic Versioning][semver] for releases. However, our
+compatibility policy only applies to the runc binary. We will make a
+best-effort attempt to reduce the impact to users that make direct use of the
+Go packages prefixed with `github.com/opencontainers/runc`, but we do not
+formally guarantee that API compatibility will be preserved.
+
+[semver]: https://semver.org/spec/v2.0.0.html
+
+### Release Cadence ###
+
+> **NOTE**: At time of writing, this proposal is still a draft as we have not
+> yet done several releases using this cadence. If you have feedback on this
+> proposal (such as how well this proposal addresses your needs as a downstream
+> packager of runc), please feel free to [open an issue][new-issue] so we can
+> discuss it further. However, the current plan is for this proposal to be
+> followed for the `1.3.0` and `1.4.0` releases in 2025.
+
+[new-issue]: https://github.com/opencontainers/runc/issues/new/choose
+
+runc follows a 6-month minor version release schedule, with the aim of releases
+happening at the end of April and October each year.
+
+The first release candidate will be created 2 months before the planned release
+date (i.e. the end of February and August, respectively), at which point the
+release branch will be created and will enter a feature freeze. No new features
+will be merged into the release branch, and large features being developed
+immediately before the feature freeze may have their merge delayed so as to not
+be included in the next release. Most releases will have two or three release
+candidates, but this may change depending on the circumstances of the release
+at the time.
+
+If a last-minute critical issue is discovered, the release may be delayed.
+However, the following release will still go according to schedule (except in
+the exceptionally unlikely scenario where the delay is 4-6 months long, in
+which case the next release is moved forward to when the subsequent release
+would have been).
+
+Here is a hypothetical release timeline to see how this works in practice:
+
+| Date       | Release      | Notes |
+| ---------- | ------------ | ----- |
+| 200X-02-28 | `1.3.0-rc.1` | `release-1.3` branch created, feature freeze. |
+| 200X-03-12 | `1.3.0-rc.2` | |
+| 200X-03-25 | `1.3.0-rc.3` | |
+| 200X-04-30 | `1.3.0`      | `1.3` release published. |
+| 200X-05-10 | `1.3.1`      | |
+| 200X-06-21 | `1.3.2`      | |
+| 200X-06-25 | `1.3.3`      | |
+| 200X-07-02 | `1.3.4`      | |
+| 200X-08-28 | `1.4.0-rc.1` | `release-1.4` branch created, feature freeze. |
+| 200X-09-15 | `1.3.5`      | Patch releases in other release branches have no impact on the new release branch. |
+| 200X-09-21 | `1.4.0-rc.2` | |
+| 200X-10-31 | `1.4.0`      | `1.4` release published. |
+| 200X-11-10 | `1.4.1`      | |
+| 200X-12-25 | `1.4.2`      | |
+
+(And so on for the next year.)
+
+### Support Policy ###
+
+In order to ease the transition between minor runc releases, previous minor
+release branches of runc will be maintained for some time after the newest
+minor release is published. In the following text, `latest` refers to the
+latest minor (non-release-candidate) runc release published; `latest-1` is the
+previous minor release branch; and `latest-2` is the minor release branch
+before `latest-1`. For example, if `latest` is `1.4.0` then `latest-1` is
+`1.3.z` and `latest-2` is `1.2.z`.
+
+ * Once `latest` is released, new features will no longer be merged into
+   `latest` and only bug and security fixes will be backported, though we will
+   be fairly liberal with what kinds of bugs will considered candidates for
+   backporting.
+
+ * `latest-1` will only receive security fixes and significant bug fixes (what
+   bug fixes are "significant" are down to the maintainer's judgement, but
+   maintainers should err on the side of reducing the number of backports at
+   this stage). Users are encouraged to start testing and consider migrating to
+   the `latest` release of runc, reporting any issues they may find.
+
+ * `latest-2` will only receive high severity security fixes (CVEs that have
+   been assessed as having CVSS score 7 or higher). At this point, users still
+   using `latest-2` would be strongly encouraged to upgrade to either `latest`
+   or `latest-1`.
+
+ * Any older releases will no longer receive any updates, and users are
+   encouraged to upgrade in the strongest possible terms, as they will not
+   receive any security fixes regardless of severity or impact.
+
+This policy only applies to minor releases of runc with major version `1`. If
+there is a runc `2.0` release in the future, this document will be updated to
+reflect the necessary changes to the support policy for the `1.y` major release
+branch of runc.