schema: Clarify kind, source, and readOnly for image manifest volumes.
schema: Clarify kind, source, and readOnly for image manifest volumes.

- Clarify what should occur based on kind.
- Add missing definition for source.
- Add missing definition for readOnly.

tklauser/spec,dgonyeo/spec,krnowak/appc-spec,alban/appc-spec,krnowak/appc-spec,stufflebear/spec,ChengTiesheng/spec,stufflebear/spec,zenlinTechnofreak/spec,yifan-gu/spec,zenlinTechnofreak/spec,iaguis/appc-spec,tklauser/spec,craimbert/spec,stufflebear/spec,apcera/appc-spec,alban/appc-spec,dustymabe/spec,endocode/appc-spec,appc/spec,appc/spec,yifan-gu/spec,endocode/appc-spec,3ofcoins/appc-spec,appc/spec,3ofcoins/appc-spec,tklauser/spec,jonboulle/spec,dgonyeo/spec,jonboulle/spec,apcera/appc-spec,squeed/spec,yifan-gu/spec,ChengTiesheng/spec,dgonyeo/spec,craimbert/spec,alban/appc-spec,marineam/spec,marineam/spec,jonboulle/spec,ChengTiesheng/spec,krnowak/appc-spec,dustymabe/spec,craimbert/spec,squeed/spec,squeed/spec,marineam/spec,dustymabe/spec,apcera/appc-spec,3ofcoins/appc-spec,zenlinTechnofreak/spec,iaguis/appc-spec,endocode/appc-spec,iaguis/appc-spec