Add changelog entry

CHANGELOG.md

@@ -1,5 +1,10 @@
 # Changelog
 
+## 2.5.1 (unreleased)
+
+### Bugfixes
+ * `ERC777`: fixed the `_send` and `_approve` internal functions not validating some of their arguments for non-zero addresses. ([#2212](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2212))
+
 ## 2.5.0 (2020-02-04)
 
 ### New features