Add 4.6 upgradeability notice

CHANGELOG.md

@@ -29,6 +29,10 @@
  * `Initializable`: add an Initialized event that tracks initialized version numbers. ([#3294](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3294))
  * `ERC2981`: make `royaltyInfo` public to allow super call in overrides. ([#3305](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3305))
 
+### Upgradeability notice
+
+* `TimelockController`: **(Action needed)** The upgrade from <4.6 to >=4.6 introduces a new `CANCELLER_ROLE` that requires set up to be assignable. After the upgrade, only addresses with this role will have the ability to cancel. Proposers will no longer be able to cancel. Assigning cancellers can be done by an admin (including the timelock itself) once the role admin is set up. To do this, we recommend upgrading to the `TimelockControllerWith46MigrationUpgradeable` contract and then calling the `migrateTo46` function.
+
 ### Breaking changes
 
 * `Governor`: Adds internal virtual `_getVotes` method that must be implemented; this is a breaking change for existing concrete extensions to `Governor`. To fix this on an existing voting module extension, rename `getVotes` to `_getVotes` and add a `bytes memory` argument. ([#3043](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3043))