Add changelog entry

CHANGELOG.md

@@ -1,5 +1,10 @@
 # Changelog
 
+## 3.0.1 (unreleased)
+
+### Bugfixes
+ * `ERC777`: fixed the `_approve` internal function not validating some of their arguments for non-zero addresses. ([#2213](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2213))
+
 ## 3.0.0 (2020-04-20)
 
 ### New features