Add RFC 2119 key words to appendix

see modelica#581

docs/6___appendix.adoc

@@ -489,3 +489,15 @@ This definition is slightly different from the usual standard definition of stat
 |_XML_
 |eXtensible Markup Language (http://www.w3.org/XML/[www.w3.org/XML], http://en.wikipedia.org/wiki/Xml[en.wikipedia.org/wiki/XML]) – An open standard to store information in text files in a structured form.
 |====
+
+## Key Words to Indicate Requirement Levels
+
+The following key words defined in https://tools.ietf.org/html/rfc2119[RFC 2119] are used to indicate requirements.
+
+MUST:: This word, or the terms "REQUIRED" or "SHALL", mean that the definition is an absolute requirement of the specification.
+
+MUST NOT:: This phrase, or the phrase "SHALL NOT", mean that the definition is an absolute prohibition of the specification.
+
+SHOULD:: This word, or the adjective "RECOMMENDED", mean that there may exist valid reasons in particular circumstances to ignore a particular item, but the full implications must be understood and carefully weighed before choosing a different course.
+
+SHOULD NOT:: This phrase, or the phrase "NOT RECOMMENDED" mean that there may exist valid reasons in particular circumstances when the particular behavior is acceptable or even useful, but the full implications should be understood and the case carefully weighed before implementing any behavior described with this label.