Add documentation for the ConditionVariable class

docs/reference/api/rtos/ConditionVariable.md

@@ -0,0 +1,62 @@
+## ConditionVariable
+
+The ConditionVariable class provides a mechanism to safely wait for or signal state changes. A common scenario when writing multithreaded code is to protect shared resources with a mutex and then release that mutex to wait for a change of that data. If you do not do this carefully, this can lead to a race condition in the code. A condition variable provides a safe solution to this problem by handling the wait for a state change, along with releasing and acquiring the mutex automatically during this waiting period.
+
+### ConditionVariable class reference
+
+[![View code](https://www.mbed.com/embed/?type=library)](https://os-doc-builder.test.mbed.com/docs/v5.7/mbed-os-api-doxy/classrtos_1_1_condition_variable.html)
+
+### ConditionVariable example
+
+Below is an example of ConditionVariable usage, where one thread is generating events every 1s, and the second thread is waiting for the events and executing an action.
+
+```
+#include "mbed.h"
+
+Mutex mutex;
+ConditionVariable cond(mutex);
+
+// These variables are protected by locking mutex
+uint32_t count = 0;
+bool done = false;
+
+void worker_thread()
+{
+    mutex.lock();
+    do {
+        printf("Worker: Count %lu\r\n", count);
+
+        // Wait for a condition to change
+        cond.wait();
+
+    } while (!done);
+    printf("Worker: Exiting\r\n");
+    mutex.unlock();
+}
+
+int main() {
+    Thread thread;
+    thread.start(worker_thread);
+
+    for (int i = 0; i < 5; i++) {
+
+        mutex.lock();
+        // Change count and signal this
+        count++;
+        printf("Main: Set count to %lu\r\n", count);
+        cond.notify_all();
+        mutex.unlock();
+
+        wait(1.0);
+    }
+
+    mutex.lock();
+    // Change done and signal this
+    done = true;
+    printf("Main: Set done\r\n");
+    cond.notify_all();
+    mutex.unlock();
+
+    thread.join();
+}
+```