Define use of null in JSON-LD

This closes #76.

Author: lanthaler

spec/latest/json-ld-syntax/index.html

@@ -384,7 +384,10 @@ <h2>How to Read this Document</h2>
       Values that are used to express one of two possible boolean states.
     </dd>
     <dt><tdef>null</tdef></dt><dd>
-      The use of the <em>null</em> value is undefined within JSON-LD.
+      Unless otherwise specified, a JSON-LD processor MUST act as if a key-value pair in the body of a JSON-LD document was never declared when the value equals <em>null</em>.
+      If <code>@value</code> or <code>@list</code> is set to <em>null</em> in expanded form, then the entire JSON object is ignored.
+      If <code>@context</code> is set to <em>null</em>, the <tref>active context</tref> is reset and when used
+      within a <tref>context</tref>, it removes any definition associated with the key, unless otherwise specified.
     </dd>
   </dl>
 </p>