docs: add release strategy (#6271)

docs/admin/release_strategy.md

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+# Release strategy
+
+## Semantic versioning
+
+Starting 0.12.0, Gogs uses [semantic versioning](https://semver.org/) for publishing releases. For example:
+
+- `0.12.0` is a minor version release.
+- `0.12.1` is the first patch release of `0.12`.
+- `0.12` indicates a series of releases for a minor version and its patch releases.
+
+Each minor release has its own release branch with prefix `release/`, e.g. `release/0.12` is the release branch for minor version 0.12.0 and all its patch releases (`0.12.1`, `0.12.2`, etc.).
+
+## Backwards compatibility
+
+### Before 0.12
+
+If you're running Gogs with any version below 0.12, please upgrade to 0.12 to run necessary migrations.
+
+### After 0.12
+
+We maintain one minor version backwards compatibility, patch releases are disregarded.
+
+For example, you should:
+
+- Upgrade from `0.12.0` to `0.13.0`.
+- Upgrade from `0.12.1` to `0.13.4`.
+- NOT upgrade from `0.12.4` to `0.14.0`.
+
+Therefore, we recommend upgrade one minor version at a time.
+
+### Running source builds
+
+If you're running Gogs with building from source code, we recommend you update at least weekly to be not fall behind and potentially miss migrations.