adding graphmap section

spec/docs/graphmap.md

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+# Assigning Triples to Named Graphs
+
+Each triple generated from an [=RML mapping=]() is placed into one or more graphs of the [=output dataset=](). Possible target graphs are the unnamed [=default graph=](), the [=IRI=]()-named [=named graphs=](), and [=blank node=]() named [=named graphs=]()
+
+Any [=subject map=]() or [=predicate-object=]() map MAY have one or more associated graph maps. They are specified in one of two ways:
+
+1. using the rml:graphMap property, whose value MUST be a [=graph map=](),
+2. using the [=constant shortcut property=]() rml:graph.
+
+Graph maps are themselves [=term maps=](). When [=RDF triples are generated=](), the set of target graphs is determined by taking into account any graph maps associated with the subject map or predicate-object map.
+
+If a [=graph map=]() generates the special IRI rml:defaultGraph, then the target graph is the [=default graph=]() of the [=output dataset=]().
+
+In the following [=subject map=]() example, all generated RDF triples will be stored in the named graph ex:DepartmentGraph.
+
+```turtle
+[]  rml:subjectMap [
+      rml:template "http://data.example.com/department/{DEPTNO}";
+      rml:graphMap [ rml:constant ex:DepartmentGraph ];
+].
+```
+This is equivalent to the following example, which uses a [=constant shortcut property=]():
+
+```turtle
+[]  rml:subjectMap [
+      rml:template "http://data.example.com/department/{DEPTNO}";
+      rml:graph ex:DepartmentGraph;
+].
+```
+In the following example, RDF triples are placed into [=IRI=]()-named [=named graphs=]() according to the job title of employees:
+```turtle
+[]  rml:subjectMap [
+      rml:template "http://data.example.com/employee/{EMPNO}";
+      rml:graphMap [ rml:template "http://data.example.com/jobgraph/{JOB}" ];
+].
+```
+
+However, in this example, RDF triples of the [=predicate-object=]() `ex:name` are placed into a [=blank node=]() named [=named graphs=]() according to the job title of employees:
+```turtle
+[]  rml:subjectMap [
+      rml:template "http://data.example.com/employee/{EMPNO}";
+    ];
+    rml:predicateObjectMap [
+      rml:predicate ex:name;
+      rml:objectMap [ rml:reference "name"; ];
+      rml:graphMap [
+        rml:reference "JOB"
+        rml:termType rr:IRI
+      ];
+    ].
+```
+