Add changelog entry

Author: thomaseizinger

protocols/ping/CHANGELOG.md

@@ -18,6 +18,14 @@
 
   [PR 2149]: https://github.com/libp2p/rust-libp2p/pull/2149/
 
+- Rename types as per [discussion 2174].
+  `Ping` has been renamed to `Behaviour`.
+  The `Ping` prefix has been removed from various types like `PingEvent`.
+  Users should prefer importing the ping protocol as a module (`use libp2p::ping;`),
+  and refer to its types via `ping::`. For example: `ping::Behaviour` or `ping::Event`.
+
+  [discussion 2174]: https://github.com/libp2p/rust-libp2p/discussions/2174
+
 # 0.30.0 [2021-07-12]
 
 - Update dependencies.