Add MUST NOT and SHALL NOT to conventions

MUST NOT and SHALL NOT means an absolute prohibition of
the spec. They should be one of requirements to judge one
implementation if it is compliant.

Signed-off-by: Ma Shimiao <[email protected]>

Author: Ma Shimiao

spec.md

@@ -29,8 +29,8 @@ 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.
+An implementation is not compliant if it fails to satisfy one or more of the MUST, MUST NOT, REQUIRED, SHALL, or SHALL NOT requirements for the protocols it implements.
+An implementation is compliant if it satisfies all the MUST, MUST NOT, REQUIRED, SHALL, and SHALL NOT requirements for the protocols it implements.
 
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