[Skiboot] stable trees and process

[Stewart Smith]

Hey all,

So, we do major releases and have stable trees, with some specified
process around doing stable (see
https://open-power.github.io/skiboot/doc/stable-skiboot-rules.html ).
Although the *reality* has been a bit more ad-hoc, with candidates for
stable being from either me talking to people on email, slack or in
(often IBM-internal) bug reports.

I'd like to change that, and have us follow the above process.

That is, *use* the "CC: stable" tag when a fix needs to get to a stable
tree.

So, if a patch needs to get to a service pack for an IBM firmware
release, it needs to get into a stable tree. (the same is probably also
true for other openpower system vendors)

-- 
Stewart Smith
OPAL Architect, IBM.