Update changelog for v1.50.0 release

Author: JLXIA

CHANGELOG.md

@@ -1,3 +1,13 @@
+# Release 1.50.0 21-04-2020
+
+- Convert 4xx ServiceControl failures to 500 (#785)
+- Add Brotli Support for NGINX in ESP (#784)
+
+# Release 1.49.0 07-04-2020
+
+- Only apply network_fail_open policy to 5xx Http status code (#779)
+- Add trace span for metadata fetching (#778)
+
 # Release 1.48.0 24-03-2020
 
 - Copy the original `Authorization` header to `X-Forwarded-Authorization` for backend auth (#772)