RFR: Section on backporting [v2]

Tue Nov 23 21:56:16 UTC 2021

On Tue, 23 Nov 2021 21:39:45 GMT, Jesper Wilhelmsson <jwilhelm at openjdk.org> wrote:

>> Guidance on backporting.
>> Also moved "Backing out a change" to the testing section where I believe it fits better. This section is unchanged.
>
> Jesper Wilhelmsson has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fixes after reviews

Marked as reviewed by kcr (no project role).

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PR: https://git.openjdk.java.net/guide/pull/66