Update 3.1 release notes

- Move the details about mandatory plugins down into the 'New Features'
  section, leaving only the bullet point in the release highlights.

- Add a link to the documentation for enabling mandatory plugins. Note
  however that the documentation has less detail than the release notes,
  so we should probably improve that before the final 3.1.0 release.

- Add brief sections about git protocol v2 and Polymer 2 in the new
  features section. These can be expanded later.

- Add release highlights bullets in the main releases index.

Change-Id: Ia1f4e2b4c2b31249d369ffff089720f8892f66f7
diff --git a/pages/site/releases/3.1.md b/pages/site/releases/3.1.md
index 0537fd8..09f252d 100644
--- a/pages/site/releases/3.1.md
+++ b/pages/site/releases/3.1.md
@@ -14,13 +14,7 @@
 
 * Mandatory plugins
 
-  It is possible to configure which plugins are mandatory.
-
-  Gerrit will fail to start if a mandatory plugin cannot be loaded.
-
-  Disabling and restarting of a mandatory plugin is rejected.
-
-  Reloading of a mandatory plugin is still possible.
+See the [New Features](#new-features) section for further details.
 
 ## Important Notes
 
@@ -74,6 +68,24 @@
 
 ## New Features
 
+### Git Protocol V2
+
+Git protocol V2 is now enabled by default. A new permission-aware ref database
+ensures that Gerrit only ever returns refs that the calling user has access to.
+
+### PolyGerrit UI Improvements
+
+Polymer is upgraded to version 2.
+
+### Mandatory Plugins
+
+It is possible to
+[configure which plugins are mandatory](http://gerrit-documentation.storage.googleapis.com/Documentation/3.1.0/config-gerrit.html#plugins.mandatory).
+Gerrit will fail to start if a mandatory plugin cannot be loaded.
+
+Disabling and restarting of a mandatory plugin is rejected, but reloading of a
+mandatory plugin is still possible.
+
 ### Replication plugin
 
 * [Issue 11196](https://bugs.chromium.org/p/gerrit/issues/detail?id=11196):
diff --git a/pages/site/releases/README.md b/pages/site/releases/README.md
index 053077d..4e2090a 100644
--- a/pages/site/releases/README.md
+++ b/pages/site/releases/README.md
@@ -20,6 +20,12 @@
 
 ## 3.1.0 (release candidate)
 
+* Support for git protocol V2
+
+* Polymer 2
+
+* Mandatory plugins
+
 [Release notes for Gerrit 3.1.0](/3.1.md)
 
 ## 3.0.0