Add example of atomic variables

atomic.cpp

+#include <array>
+#include <thread>
+#include <iostream>
+
+int main() {
+    constexpr auto n_threads = 10u;
+    std::atomic<int> counter = {0};
+    std::array<std::thread, n_threads> threads;
+
+    const auto increment = [](std::atomic<int>& i) {
+        // We don't care what order the the threads perform their increment operations.
+        i.fetch_add(1, std::memory_order_relaxed);
+    };
+
+    for (auto i = 0u; i < n_threads; ++i) {
+        threads[i] = std::thread(increment, std::ref(counter));
+    }
+
+    for (auto i = 0u; i < n_threads; ++i) {
+        threads[i].join();
+    }
+
+    std::cout << "Final value: " << counter << std::endl;
+
+    return 0;
+}
+