docs(dev): Add backport considerations to development process

developer_manual/getting_started/development_process.rst

@@ -40,6 +40,50 @@ Bugfixes
 
 If a contribution fixes a bug that also affects older Nextcloud or app releases, it may qualify for a *backport*. Backporting a fix means applying the change on an older version of the code. Git calls this operation *cherry picking*.
 
+Whenever a critical bug (i.e. security vulnerability) is fixed, it is backported to all applicable major releases and - once merged - published in the next set of maintenance releases for all still supported majors (e.g. 28.0.3 -> 28.0.4). 
+
+Backporting Considerations
+**************************
+
+Major releases that have already been published do not need necessarily need every bug fix. Deciding what to backport is not obvious. There is some judgement involved here.
+
+Here are some things to consider:
+
+- Any major release that has not reached end-of-life status usually receives these backported fixes.
+- Backporting even the simplest changes has some level of risk.
+- Differences among stable branches - including of shipped and third-pary apps - means there are additional variables outside of the main branch (or even versus the latest stable).
+- Fixes often do not have a lot of time in the field (< 4 weeks and it's possible no one has directly interacted with the new code outside of the original developer).
+
+Showstoppers (never backport things that cause these):
+
+- API changes [security related matters will be handled on a case-by-case basis since they have a unique priority]
+
+When assessing whether a bug is critical enough to backport, here are some possible questions to ask yourself:
+
+- Is this really a bug fix? Or is it more an enhancement or a general improvement?
+- Is it even applicable to a previously published major release train?
+- Is that major release train still supported?
+- Is it a security vulnerability? [Yes, backport it without question.]
+- How well can the change be tested?
+- How confident are we in the fix?
+- Is the bug likely to impact many users/environments?
+- Is there *any* likelihood this change could inadvertenly introduce data loss?
+- Is there *any* likelihood this change could inadvertenly introduce a security matter?
+- How "hairy" is the change in general?
+- Are we willing to support the backport if the change breaks something unexpected in a prior release?
+- Can the change be backported as-is or will it require significant reworking?
+- Is the bug causing a lot of support requests or bug reports?
+- Does the main tracking Issue have a lot of upvotes/subscribers/comments?
+- Is it possible to take a "wait and see" attitude about backporting (i.e. continue to test the fix in main/master branch and wait one maintenance cycle to re-evaluate and only backport if further data from the field suggests its important enough and/or low-risk enough to do so)?
+
+Use your best judgement. 
+
+If appropriate, mention any major concerns in the backport PR so other code reviewers can consider them.
+
+Ideally, when triggering/requesting a backport, also explain *why* the backport is necessary (if it's not obvious). This will further help reviewers.
+
+TLDR: Backporting bug fixes to older versions of code can have unintended side effects. Not every fix needs to be backported. Use caution.
+
 Automatic Backport
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