agl-compositor.md: Add some notes about v2 and v3 protocol update
This includes some notes which might be useful for people to reason
about why these updates are happening.
Bug-AGL: SPEC-4528
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: I526af1b1b12f9bfbeae106d6ec4a436de172d318
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/documentation/+/27999
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Tested-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>