adds text describing the upper-case keywords used in the spec

Signed-off-by: Mike Brown <[email protected]>

Author: mikebrow

README.md

@@ -16,6 +16,11 @@ Table of Contents
   - [Linux Specific Runtime](runtime-linux.md)
 - [Implementations](implementations.md)
 
+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.)
+
 # Use Cases
 
 To provide context for users the following section gives example use cases for each part of the spec.