added examples

doc/examples/at_json_pointer_const.cpp

@@ -0,0 +1,23 @@
+#include <json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    // create a JSON value
+    json j =
+    {
+        {"number", 1}, {"string", "foo"}, {"array", {1, 2}}
+    };
+
+    // read-only access
+
+    // output element with JSON pointer "/number"
+    std::cout << j.at("/number"_json_pointer) << '\n';
+    // output element with JSON pointer "/string"
+    std::cout << j.at("/string"_json_pointer) << '\n';
+    // output element with JSON pointer "/array"
+    std::cout << j.at("/array"_json_pointer) << '\n';
+    // output element with JSON pointer "/array/1"
+    std::cout << j.at("/array/1"_json_pointer) << '\n';
+}