Add platforms documentation

This adds the documentation about supported Kubernetes platforms to our
repository.

Signed-off-by: Sascha Grunert <sgrunert@suse.com>

release-engineering/platforms.md

@@ -0,0 +1,65 @@
+# Supported Platforms
+
+The Kubernetes release process produces [artifacts](./artifacts.md) for
+different architectures and operating systems. We consider the combination of
+architecture (`GOARCH`) and operating system (`GOOS`) as "platforms". Target of
+this document is to outline different categories of platforms as well as guiding
+through their graduation criteria.
+
+## Tiers
+
+Support for different platforms are organized into three tiers, whereas each
+comes with a different set of guarantees.
+
+### Tier 1
+
+Tier 1 platforms are considered as "guaranteed to work". To achieve this, they
+have to fulfil the following criteria:
+
+- **All** official binary releases are provided for the platform. This includes
+  container images as well as deb and rpm packages. Building the artifacts is
+  integrated in the release process.
+- Continuous Integration is set up to run tests for the platform. Necessary
+  tests are defined by SIG Release and usually correspond to the
+  [`blocking`](https://testgrid.k8s.io/sig-release-master-blocking) and
+  [`informing`](https://testgrid.k8s.io/sig-release-master-informing) testgrid
+  dashboards.
+- Documentation about the usage of artifacts for the platform is available.
+
+### Tier 2
+
+Tier 2 platforms are considered as "guaranteed to build". To achieve this, they
+have to fulfil the following criteria:
+
+- **Any** official binary release is provided for the platform. Building the
+  artifacts is integrated in the release process.
+- Automated testing is not or only partially setup.
+
+It may be possible that single features are not available for a certain
+platform.
+
+### Tier 3
+
+Tier 3 platforms are those which the Kubernetes codebase has support for, but
+which are not built or tested automatically. This means the binary artifacts may
+not work as intended.
+
+- Official builds are not available.
+- Automated testing is not setup.
+
+## Currently available Kubernetes platforms
+
+The following table defines the current setup of available Kubernetes platforms:
+
+| Platform        |       Tier 1       |       Tier 2       |       Tier 3       |
+| --------------- | :----------------: | :----------------: | :----------------: |
+| `amd64-linux`   | :heavy_check_mark: |                    |                    |
+| `arm64-linux`   |                    | :heavy_check_mark: |                    |
+| `arm-linux`     |                    | :heavy_check_mark: |                    |
+| `amd64-darwin`  |                    | :heavy_check_mark: |                    |
+| `386-linux`     |                    | :heavy_check_mark: |                    |
+| `ppc64le-linux` |                    | :heavy_check_mark: |                    |
+| `s390x-linux`   |                    | :heavy_check_mark: |                    |
+| `386-windows`   |                    | :heavy_check_mark: |                    |
+| `amd64-windows` |                    | :heavy_check_mark: |                    |
+| otherwise       |                    |                    | :heavy_check_mark: |