Add doc/stable-skiboot-rules.txt documenting stable tree

Signed-off-by: Stewart Smith <stewart@linux.vnet.ibm.com>

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diff --git a/doc/stable-skiboot-rules.txt b/doc/stable-skiboot-rules.txt
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+Stable Skiboot tree/releases
+----------------------------
+
+If you're at all familiar with the Linux kernel stable trees, this should
+seem fairly familiar.
+
+The purpose of a -stable tree is to give vendors a stable base to create
+firmware releases from and to incorporate into service packs. New stable
+releases contain critical fixes only.
+
+As a general rule, on the most recent skiboot release gets a maintained
+-stable tree. If you wish to maintain an older tree, speak up! For example,
+with my IBMer hat on, we'll maintain branches that we ship in products.
+
+What patches are accepted?
+--------------------------
+
+- Patches must be obviously correct and tested
+  - A Tested-by signoff is *important*
+- A patch must fix a real bug
+- No trivial patches, such fixups belong in main branch
+- Not fix a purely theoretical problem unless you can prove how
+  it's exploitable
+- The patch, or an equivalent one, must already be in master
+  - Submitting to both at the same time is okay, but backporting is better
+
+HOWTO submit to stable
+----------------------
+Two ways:
+1) Send patch to the skiboot@ list with "[PATCH stable]" in subject
+   - This targets the patch *ONLY* to the stable branch.
+      - Such commits will *NOT* be merged into master.
+   - Use this when:
+     a) cherry-picking a fix from master
+     b) fixing something that is only broken in stable
+     c) fix in stable needs to be completely different than in master
+     If b or c: explain why.
+   - If cherry-picking, include the following at the top of your
+     commit message:
+        commit <sha1> upstream.
+   - If the patch has been modified, explain why in description.
+
+2) Add "Cc: stable" above your Signed-off-by line when sending to skiboot@
+   - This targets the patch to master and stable.
+   - You can target a patch to a specific stable tree with:
+       Cc: stable # 5.1.x
+     and that will target it to the 5.1.x branch.
+   - You can ask for prerequisites to be cherry-picked:
+       Cc: stable # 5.1.x 55ae15b Ensure we run pollers in cpu_wait_job()
+       Cc: stable # 5.1.x
+     Which means:
+       1) please git cherry-pick 55ae15b
+       2) then apply this patch to 5.1.x".
+
+Trees
+-----
+- https://github.com/open-power/skiboot/tree/stable
+  git@github.com:open-power/skiboot.git (stable branch)
+
+- Some stable versions may last longer than others
+  - So there may be stable, stable-5.1 and stable-5.0 at any one time being
+    actively maintained.