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.

從 devguide 中現有的、關於將修正從 feature (master) branch 回移植到維護分支的章節開始。檢視目前的指引，並記錄開發者應如何決定是否需要回移植，包括延遲修正或過早送出修正的風險；當該政策清楚到足以引導這些決策時，即視為完成。