adding benchmarks and readme

lectures/optimization/map_erase/map_erase_slow.cpp

@@ -0,0 +1,20 @@
+#include <iostream>
+#include <map>
+
+int main() {
+    const int N = 1000000;
+    std::map<int, int> m;
+
+    // fill the map with some values
+    for (int i = 0; i < N; ++i) {
+        m[i] = i;
+    }
+
+    // erase by key
+    for (int i = 0; i < N; ++i) {
+        m.erase(i);
+    }
+
+    return 0;
+}
+