image-spec: add RFC boiler plate

[philips]

Similar to https://github.com/opencontainers/runtime-spec#open-container-runtime-specification boiler plate about MUST, MUST NOT, etc.

cc @RobDolinMS and @JLB13

[RobDolinMS]

(Howard Huang from Huawei also volunteered to help)

[RobDolinMS]

Opened PR #207 to address this

[wking]

#207 has landed, so this can be closed. Not sure why GitHub is not sending me email about these ;).