update changelog to indicate pre-release changes
# Changelog
## [unreleased]
+
+## `4.0.0-pre.1` [07-23-2021]
* Implements new `LazyOption` type under `unstable` feature. Similar to `Lazy` but is optional to set a value. [PR 444](https://github.com/near/near-sdk-rs/pull/444).
* Move type aliases and core types to near-sdk to avoid coupling. [PR 415](https://github.com/near/near-sdk-rs/pull/415).
* Implements new `Lazy` type under the new `unstable` feature which is a lazily loaded storage value. [PR 409](https://github.com/near/near-sdk-rs/pull/409).
