Document release management rules

Author: pabzm

RELEASE_MANAGEMENT.md

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+# Release management
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+Roundcubemail releases follow the rules of [semantic versioning](https://semver.org). As public API in the sense of that document we consider [the plugin API](https://github.com/roundcube/roundcubemail/wiki/Plugin-API) and the upgrade process.
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+We fully maintain and support only the latest released version of Roundcubemail.
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+Only security and similarly critical fixes will be published for the previous minor version, too.
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+(As an example: Imagine the latest release to be version 1.2.3, and the latest release before 1.2.0 to have been 1.1.5. Then, a security fix would be released in a version 1.2.4, as well as a version 1.1.6. A new feature or uncritical bug fix would only be released in a version 1.2.4.)
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+For new minor or major version, we might release beta versions in advance, but we don't promise those.
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+There is no guarantee that a change in the "master" branch will be included in the next release.
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+Exceptions of these rules may happen, but only for good reasons.