Add beacon proxy changelog entry

CHANGELOG.md

@@ -1,5 +1,9 @@
 # Changelog
 
+## Unreleased
+
+ * Add beacon proxy. ([#2411](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2411))
+
 ## Unreleased
 
  * Now supports both Solidity 0.6 and 0.7. Compiling with solc 0.7 will result in warnings. Install the `solc-0.7` tag to compile without warnings.