Added thread-join

docs/API.md

@@ -1122,6 +1122,7 @@ This section is an alphabetic listing of all the functions, objects, and macros
 [`test-vars`](api/scheme/cyclone/test#test-vars)
 [`test`](api/scheme/cyclone/test#test)
 [`third`](api/srfi/1#third)
+[`thread-join!`](api/srfi/18#thread-join)
 [`thread-name`](api/srfi/18#thread-name)
 [`thread-sleep!`](api/srfi/18#thread-sleep)
 [`thread-specific-set!`](api/srfi/18#thread-specific-set)

docs/api/srfi/18.md

@@ -24,6 +24,7 @@ present in this implementation will fail to compile.
 - [`thread-sleep!`](#thread-sleep)
 - [`thread-yield!`](#thread-yield)
 - [`thread-terminate!`](#thread-terminate)
+- [`thread-join!`](#thread-join)
 - [`mutex?`](#mutex)
 - [`make-mutex`](#make-mutex) 
 - [`mutex-lock!`](#mutex-lock)
@@ -92,6 +93,12 @@ The current thread exits the running state as if its quantum had expired.
 
 Immediately abort the current thread.
 
+# thread-join!
+
+    (thread-join! thread)
+
+The current thread waits until `thread` terminates.
+
 # mutex?
 
     (mutex? obj)