Update 2.15 release notes to mention private changes feature

Add the initial description of the feature and entries in the REST API
section for the new endpoints.

Change-Id: I3dd87a60fc925004a07553e8b1581746151f3d9e
diff --git a/releases/2.15.md b/releases/2.15.md
index 03b9d27..bbf0628 100644
--- a/releases/2.15.md
+++ b/releases/2.15.md
@@ -4,6 +4,8 @@
 
 ## Release Highlights
 
+* Private Changes
+
 ## Important Notes
 
 ### Schema Changes
@@ -16,6 +18,22 @@
 
 ## New Features
 
+### Private Changes
+
+Changes can be
+[marked as private](http://gerrit-documentation.storage.googleapis.com/Documentation/2.15/intro-user.html#private-changes),
+meaning they are only visible to the owner, reviewers, and users with the
+["View Private Changes"](http://gerrit-documentation.storage.googleapis.com/Documentation/2.15/access-control.html#category_view_private_changes)
+permission.
+
+### REST API
+
+#### Changes
+
+* [Mark Private](http://gerrit-documentation.storage.googleapis.com/Documentation/2.15/rest-api-changes.html#mark-private)
+
+* [Unmark Private](http://gerrit-documentation.storage.googleapis.com/Documentation/2.15/rest-api-changes.html#unmark-private)
+
 ## Bugfixes
 
 ## Dependency Updates
diff --git a/releases/README.md b/releases/README.md
index e4a73ac..624bb05 100644
--- a/releases/README.md
+++ b/releases/README.md
@@ -6,6 +6,8 @@
 
 ## 2.15 *(In development)* {#2.15}
 
+* Private Changes
+
 [Release notes for Gerrit 2.15](/releases/2.15.md)
 
 ## 2.14 *(In development)* {#2.14}