Update 2.15 release notes to mention new workflows

Create a new top level item "New Workflows".  Move the existing section
about private changes underneath it, and also add sections about the new
features: ignore, mute, and work-in-progress.

Change-Id: Icfeabd24dd9d2d0d6a11a974583237b4bfe78ab5
diff --git a/releases/2.15.md b/releases/2.15.md
index bbf0628..7ddf88c 100644
--- a/releases/2.15.md
+++ b/releases/2.15.md
@@ -4,7 +4,7 @@
 
 ## Release Highlights
 
-* Private Changes
+* New change workflows
 
 ## Important Notes
 
@@ -18,7 +18,9 @@
 
 ## New Features
 
-### Private Changes
+### New Workflows
+
+#### Private Changes
 
 Changes can be
 [marked as private](http://gerrit-documentation.storage.googleapis.com/Documentation/2.15/intro-user.html#private-changes),
@@ -26,6 +28,16 @@
 ["View Private Changes"](http://gerrit-documentation.storage.googleapis.com/Documentation/2.15/access-control.html#category_view_private_changes)
 permission.
 
+#### Ignore and Mute Changes
+
+Changes can be
+[ignored or muted](http://gerrit-documentation.storage.googleapis.com/Documentation/2.15/intro-user.html#ignore)
+to reduce email notifications and keep dashboards cleaner.
+
+#### Work-In-Progress
+
+Changes can be switched between "work in progress" and "ready to review".
+
 ### REST API
 
 #### Changes