add graph module description

src/main/kotlin/org/objectionary/ddr/graph/doc/README.md

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+### Inheritance graph
+
+Graph module takes a collection of `org.w3c.dom.Document` objects and constructs an inheritance graph, connecting all of them.
+With this graph you can access the list of all the attributes present in an object in constant time.
+
+#### Example:
+
+Consider the following EO program.
+
+```
+[] > animal
+  [] > live
+    [] > eat
+      stdout > @
+        "Animal is eating"
+    stdout > @
+      "Animal is alive"
+  [t] > talk
+    live > @
+
+[] > mouse
+  animal > @
+  [] > mouse_live
+    ^.live > @
+
+[] > cat
+  animal > @
+  [m] > meow
+    ^.talk "Meow" > @
+```
+
+The resulting graph will look like below. Each node is represented with a rectangle. All the attributes
+of the node are listed in the rectangle.
+
+![](diag.drawio.png)