Documentation about backward compatibility

Signed-off-by: Fabio José <fabiojose@gmail.com>

README.md

@@ -51,6 +51,20 @@ Javascript SDK for CloudEvents
  */
 var cloudevent01 = new Cloudevent();
 
+/*
+ * Implemented using [Builder Design Pattern](https://en.wikipedia.org/wiki/Builder_pattern)
+ */
+cloudevent01
+  .type("com.github.pull.create")
+  .source("urn:event:from:myapi/resourse/123");
+
+/*
+ * Backward compatibility by injecting methods from spec implementation to `Cloudevent`
+ *   See how [here](lib/specs/spec_0_1.js#L17)
+ */
+cloudevent01
+ .eventTypeVersion("1.0");
+
 /*
  * Constructs an instance with:
  *   - Spec 0.2
@@ -58,4 +72,12 @@ var cloudevent01 = new Cloudevent();
  */
 var cloudevent02 = new Cloudevent(Cloudevent.specs['0.2']);
 
+/*
+ * Different specs, but the same API.
+ */
+cloudevent02
+  .type("com.github.pull.create")
+  .source("urn:event:from:myapi/resourse/123");
+
+
 ```

test/cloudevent_spec_0_1.js

@@ -27,6 +27,13 @@ describe("CloudEvents Spec 0.1 - JavaScript SDK", () => {
       });
     });
 
+    describe("Backward compatibility", () => {
+      it("should have 'eventTypeVersion'", () => {
+        cloudevent.eventTypeVersion("1.0");
+        expect(cloudevent.format()).to.have.property('eventTypeVersion');
+      });
+    });
+
   });
 
 });