Update readme

Author: Stjepan Glavina

README.md

@@ -11,10 +11,12 @@ https://docs.rs/async-lock)
 
 Async locking primitives.
 
-This crate provides two primitives:
+This crate provides the following primitives:
 
+* `Barrier` - enables tasks to synchronize all together at the same time.
 * `Mutex` - a mutual exclusion lock.
 * `RwLock` - a reader-writer lock, allowing any number of readers or a single writer.
+* `Semaphore` - limits the number of concurrent operations.
 
 ## License