doc: stable tree rules

In co-operation with Vasant, here's a start describing our stable
process.

Signed-off-by: Stewart Smith <stewart@linux.ibm.com>
diff --git a/doc/process/index.rst b/doc/process/index.rst
index 4efe358..1d0af3e 100644
--- a/doc/process/index.rst
+++ b/doc/process/index.rst
@@ -6,3 +6,4 @@
    CONTRIBUTING.md
    KernelTree.rst
    building-with-ci
+   stable-rules.rst