add release notes

Release.md

@@ -0,0 +1,8 @@
+### New
+
+* Added config `bandwidth_limit_mode` in frpc, default value is `client` which is current behavior. Optional value is `server`, to enable bandwidth limit in server. The major aim is to let server plugin has the ability to modify bandwidth limit for each proxy.
+
+### Improve
+
+* `dns_server` supports ipv6.
+* frpc supports graceful shutdown for protocol `quic`.