Provide guidelines for backporting

[ned-deily]

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.

[hugovk]

@JelleZijlstra has written some notes at https://jellezijlstra.github.io/cpython#backporting

Related: #1069 asking about backporting docs.