Provide guidelines for backporting

The devguide currently gives information about how to do a backport of a fix from the feature (master) branch to maintenance branches but I see very little in the way of guidance on how to decide if a backport is needed. The question has come up with core developers not really knowing what our policy, resulting in either a potentially beneficial fix not reaching users until one (even two) feature releases have passed or a fix being sent out prematurely in current maintenance releases introducing risk or, worse, an unnecessary incompatibility.

devguide にある、feature (master) branch から maintenance branches へ修正をバックポートする既存のセクションから始めてください。現在のガイダンスを確認し、バックポートが必要かどうかを開発者がどのように判断すべきかを文書化してください。これには、修正を遅らせる場合や時期尚早に送る場合のリスクも含めます。これらの判断を導けるほどポリシーが明確になれば完了です。