Add changelog entry about support for both 0.6 and 0.7

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## 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.
  * `Address`: added `functionStaticCall` and `functionDelegateCall`, similar to the existing `functionCall`. ([#2333](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2333))
  * `TimelockController`: added a contract to augment access control schemes with a delay. ([#2364](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2364))
  * `EnumerableSet`: added `Bytes32Set`, for sets of `bytes32`. ([#2395](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2395))