add v0.6.1 release notes (#919)

* add v0.6.1 release notes

* apply review comments

* rename to release-notes

* Update release-notes.txt

release-notes.txt

@@ -0,0 +1,17 @@
+Release v0.6.1 release notes.
+
+New features:
+    The 'paictl' tool: Introducing paictl, the deployment/management tool with the functionalities of image building, service start/stop, k8s bootup/clean, and configuration generation.
+    Single-box deployment: Support single-box deployment for evaluation purpose.
+    New UI for user management: Now the console for administrators to manage PAI users has got a new UI.
+    Documentation: Significant changes on documents -- more comprehensive, more structured, and easier to follow.
+
+Improvements:
+    Faster loading of the job list UI: Now the page gets 5x faster than before when loading its content.
+
+Bug fixes: (Omitted here)
+
+Known issues:
+    #827 Deploy PAI master and worker on the same node may lead to resource competition.
+    #813 Still in investigation. Install PAI on some old kernel may fail.
+    #713 Yarn may not use all the resource shown on the PAI dashboard, due to configuration issues.