Update CHANGES.md

Update CHANGES.md to have fields for 2.48.0 release

CHANGES.md

@@ -49,6 +49,37 @@
 
 * ([#X](https://github.com/apache/beam/issues/X)).
 -->
+# [2.48.0] - Unreleased
+
+## Highlights
+
+* New highly anticipated feature X added to Python SDK ([#X](https://github.com/apache/beam/issues/X)).
+* New highly anticipated feature Y added to Java SDK ([#Y](https://github.com/apache/beam/issues/Y)).
+
+## I/Os
+
+* Support for X source added (Java/Python) ([#X](https://github.com/apache/beam/issues/X)).
+
+## New Features / Improvements
+
+* X feature added (Java/Python) ([#X](https://github.com/apache/beam/issues/X)).
+
+## Breaking Changes
+
+* X behavior was changed ([#X](https://github.com/apache/beam/issues/X)).
+
+## Deprecations
+
+* X behavior is deprecated and will be removed in X versions ([#X](https://github.com/apache/beam/issues/X)).
+
+## Bugfixes
+
+* Fixed X (Java/Python) ([#X](https://github.com/apache/beam/issues/X)).
+
+## Known Issues
+
+* ([#X](https://github.com/apache/beam/issues/X)).
+
 
 # [2.47.0] - Unreleased