add link to sdk doc

docs/modules/sdk/nav.adoc

@@ -1,4 +1,4 @@
-* SDK
+* xref:sdk.adoc[SDK]
 ** Video Tutorials
 *** SDK Use Case Tutorials
 ****  link:http://www.youtube.com/watch?v=-OzRZscLlHY[Demo 1 (Quixote demo)]