[sdk] Add placeholders for 2.13 changelog entries

Change-Id: I27a070764d5a0551e6a42000201ff9bd7ee2ab7f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/183690
Reviewed-by: William Hesse <whesse@google.com>
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 75ddaaa..afa88f5 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,16 @@
+## 2.13.0
+
+### Language
+
+### Core libraries
+
+### Dart VM
+
+### Tools
+
+#### Dartanalyzer
+
+
 ## 2.12.0
 
 ### Language