doc/src/backport_policy: init (#2170)

Link: https://github.com/nix-community/stylix/pull/2170

Reviewed-by: Daniel Thwaites <danth@danth.me>
Reviewed-by: 0xda157 <da157@voidq.com>
(cherry picked from commit e28b20aca6)

.github/PULL_REQUEST_TEMPLATE.md

@@ -13,4 +13,4 @@ drastically accelerate the reviewing and merging process of this PR.
 - [ ] Theming changes adhere to the [Stylix style guide](https://nix-community.github.io/stylix/styling.html)
 - [ ] Changes have been [tested locally](https://nix-community.github.io/stylix/modules.html#development-setup)
 - [ ] Changes have been [tested in testbeds](https://nix-community.github.io/stylix/testbeds.html)
-- [ ] Each commit in this PR is suitable for backport to the current stable branch
+- [ ] Each commit in this PR is [suitable for backport](https://nix-community.github.io/stylix/backport_policy.html)

doc/src/SUMMARY.md

@@ -13,6 +13,7 @@
 - [Adding modules](modules.md)
 - [Testbeds](testbeds.md)
 - [Style guide](styling.md)
+- [Backport Policy](backport_policy.md)
 
 # Reference
 

doc/src/backport_policy.md

@@ -0,0 +1,12 @@
+# Backport Policy
+
+The backport policy defines what changes from the unstable (master) branch are
+applied to the current stable (`release-*.*`) branch.
+
+To reduce maintenance efforts and improve stability on stable branches, security
+fixes, bug fixes, and CI changes are backported, while new features, modules,
+and theme improvements are not backported. Upstream changes causing theming
+issues are considered a bug.
+
+New modules and theme improvements may be backported when explicitly requested
+and backporting is trivial.