add RW lock example

Author: Vladimir Kotal

threads.tex

@@ -1187,6 +1187,7 @@
 reader threads. E.g. if a lock is owned by writer and some other thread
 calls function \funnm{pthread\_rwlock\_rdlock} and there is at least one thread
 waiting in \funnm{pthread\_rwlock\_wrlock}, the writer will be given precedence.
+See \example{pthreads/pthread-rwlock.c}.
 \item There is maximal count of locking operations in given implementation
 (inferred from the type that holds the lock count). If the maximum is reached
 \funnm{pthread\_rwlock\_rdlock} returns the \texttt{EAGAIN} error.