RFR: Section on backporting

[Kevin Rushforth]

On Sat, 20 Nov 2021 02:03:51 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.

src/index.md line 1492:

> 1490: In general there is no need to create backport issues in JBS manually. All work that is done in JBS in preparation for a backport (requesting approvals etc) is done in the main issue. The backport issue will be created automatically by the bots when you integrate the change to the source code repository.
> 1491: 
> 1492: There can be cases where it is motivated to create a backport issue before the fix is done. In these cases set the fix version of the backport to `N-pool`, where `N` is the release train the backport is targeting. E.g. `17-pool`

You might want to add a sentence saying that if a backport record is created ahead of time, all work done in JBS is still done in the main issue.

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