Reference RFC 2119

[wking]

This spec uses RFC 2119's MUST, etc., but does not include a reference to the RFC. We need someone to PR something like opencontainers/runtime-spec#260.

[RobDolinMS]

@wking I think I beat you to this by about an hour with PR #207 ;)

If you agree, please close. Thanks.

[wking]

Yup, thanks :).