Add semaphore pattern implementation. (#1257)

Semaphore pattern implementation with timeout of lock/unlock operations based on channel and context.

README.md

@@ -486,6 +486,7 @@ Please take a quick gander at the [contribution guidelines](https://github.com/a
 * [goworker](https://github.com/benmanns/goworker) - goworker is a Go-based background worker
 * [grpool](https://github.com/ivpusic/grpool) - Lightweight Goroutine pool.
 * [pool](https://github.com/go-playground/pool) - a limited consumer goroutine or unlimited goroutine pool for easier goroutine handling and cancellation.
+* [semaphore](https://github.com/kamilsk/semaphore) - Semaphore pattern implementation with timeout of lock/unlock operations based on channel and context.
 * [tunny](https://github.com/Jeffail/tunny) - A goroutine pool for golang.