added first insert functions

doc/examples/insert.cpp

@@ -0,0 +1,16 @@
+#include <json.hpp>
+
+using namespace nlohmann;
+
+int main()
+{
+    // create a JSON array
+    json v = {1, 2, 3, 4};
+
+    // insert number 10 before number 3
+    auto new_pos = v.insert(v.begin() + 2, 10);
+
+    // output new array and result of insert call
+    std::cout << *new_pos << '\n';
+    std::cout << v << '\n';
+}