Add new guidelines to API compatibility (#2061)
Add new guidelines to API compatibility (#2061)

* Add new guidelines to API compatibility

* Update GUIDELINES.md

Co-authored-by: Neeraj Poddar <a149eeee45eb403766430430b9226b2aee36c00e@gmail.com>

* Add exceptions clause

Co-authored-by: Neeraj Poddar <a149eeee45eb403766430430b9226b2aee36c00e@gmail.com>
istio/api,istio/api,istio/api,istio/api