[PATCH] docs: Document backport criteria

[Stephen Finucane]

Explain why we don't want to be in the business of backport certain
patches, in the long run. It took me a while to put this into words but
I was helped by a similar discussion ongoing in the OpenStack community
at the moment [1].

[1] http://lists.openstack.org/pipermail/openstack-discuss/2019-May/006220.html

Signed-off-by: Stephen Finucane <stephen at that.guru>
Cc: Daniel Axtens <dja at axtens.net>
---
 docs/development/releasing.rst | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/docs/development/releasing.rst b/docs/development/releasing.rst
index 86cacb3a..8bb6b314 100644
--- a/docs/development/releasing.rst
+++ b/docs/development/releasing.rst
@@ -115,3 +115,30 @@ when committing::
 
 When enough patches have been backported, you should release a new **PATCH**
 release.
+
+Backport criteria
+~~~~~~~~~~~~~~~~~
+
+We consider bug fixes and security updates to the Patchwork code itself valid
+for backporting, along with fixes to documentation and developer tooling. We do
+not, however, consider the following backportable:
+
+Features
+  Backporting features is complicated and introduces instability in what is
+  supposed to be stable release. If new features are required, users should
+  update their Patchwork version.
+
+API changes
+  Except for bug fixes that resolve 5xx-class errors or fix security issues.
+  This also applies to API versions.
+
+Requirement changes
+  Requirements on a stable branch are provided as a "snapshot in time" and, as
+  with features, should not change so as to prevent instability being introduced
+  in a stable branch. In addition, stable requirements are not a mechanism to
+  alert users to security vulnerabilities and should not be considered as such.
+  Users of stable branches should either rely on distro-provided dependencies,
+  which generally maintain a snapshot-in-time fork of packages and selectively
+  backport fixes to them, or manage dependencies manually. In cases, where using
+  a distro-provided package necessitates minor changes to the Patchwork code,
+  these can be discussed on a case-by-case basis.
-- 
2.21.0