spec: Restore compliance language

This landed in 836fb1c ([ReadMe] Add Compliance Language, 2016-10-11,
opencontainers#432) but was removed in 27508e2 (Clean-up spec to remove
non-spec-ish items, 2016-11-14, opencontainers#458), likely accidentally during a
rebase.

Signed-off-by: W. Trevor King <wking@tremily.us>

spec.md

@@ -25,6 +25,9 @@ The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "S
 
 The key words "unspecified", "undefined", and "implementation-defined" are to be interpreted as described in the [rationale for the C99 standard][c99-unspecified].
 
+An implementation is not compliant if it fails to satisfy one or more of the MUST, REQUIRED, or SHALL requirements for the protocols it implements.
+An implementation is compliant if it satisfies all the MUST, REQUIRED, and SHALL requirements for the protocols it implements.
+
 # Overview
 
 At a high level the image manifest contains metadata about the contents and dependencies of the image including the content-addressable identity of one or more [filesystem layer changeset](layer.md) archives that will be unpacked to make up the final runnable filesystem.