Merge pull request opencontainers#374 from mrunalp/pr373

Additional language for conformance statement

Author: vbatts

README.md

@@ -21,6 +21,9 @@ Table of Contents
 
 In the specifications in the above table of contents, the keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in [RFC 2119](http://tools.ietf.org/html/rfc2119) (Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997).
 
+An implementation is not compliant if it fails to satisfy one or more of the MUST or REQUIRED requirements for the protocols it implements. 
+An implementation that satisfies all the MUST or REQUIRED and all the SHOULD requirements for its protocols is said to be "unconditionally compliant".
+
 # Use Cases
 
 To provide context for users the following section gives example use cases for each part of the spec.