async-rwlock 1.2.0

# Version 1.2.0

- Add a note to `RwLock::read()` about concurrent attempts to acquire a write lock.
- Add `RwLock::upgradable_read()`, `RwLock::try_upgradable_read()`, and `RwLockUpgradableGuard`.

# Version 1.1.0

- Allow `T: ?Sized`.

# Version 1.0.1

- Clarify the fairness strategy in the docs.

# Version 1.0.0

- Initial version