📝 added documentation

doc/examples/json_pointer__operator_add.cpp

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+#include <iostream>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    // create a JSON pointer
+    json::json_pointer ptr("/foo");
+    std::cout << ptr << '\n';
+
+    // apppend a JSON Pointer
+    ptr /= json::json_pointer("/bar/baz");
+    std::cout << ptr << '\n';
+
+    // append a string
+    ptr /= "fob";
+    std::cout << ptr << '\n';
+
+    // append an array index
+    ptr /= 42;
+    std::cout << ptr << std::endl;
+}