Add description of variableMeasured to SPECIFICATION.md

SPECIFICATION.md

@@ -120,6 +120,9 @@ For instance, a "comment" on an experiment will exist as a `@type: Comment` node
 ### Specific fields
 
 * `contentSize`: this term is loosely defined by Schema.org. In a .eln it is a string with the number of bytes. See "Example File" section below. It contains no units.
+* `variableMeasured`: this term is interpreted more loosely for .eln files than by Schema.org, as consisting of `@type: PropertyValue` nodes that represent not just variables measured, but also variables specified for a `@type: Dataset` node (e.g. flexible metadata).
+  * The `identifier` for a `@type: PropertyValue` node can be set to an IRI (e.g. the URL for an ontology entry, such as http://purl.org/dc/terms/instructionalMethod) for specifying the meaning of this node.
+  * Nested metadata (e.g. arrays or key-value pairs) can be represented by using `.` as a separator in their `propertyID`, e.g. `temperatures.0` or `configuration.pressure.set_value`.
 
 #### Example Dataset