add online examples

doc/examples/README.cpp

@@ -0,0 +1,31 @@
+#include <json.hpp>
+
+using namespace nlohmann;
+
+int main()
+{
+    // create a JSON object
+    json j = {
+        {"pi", 3.141},
+        {"happy", true},
+        {"name", "Niels"},
+        {"nothing", nullptr},
+        {"answer", {
+            {"everything", 42}
+        }},
+        {"list", {1, 0, 2}},
+        {"object", {
+            {"currency", "USD"},
+            {"value", 42.99}
+        }}
+    };
+    
+    // add new values
+    j["new"]["key"]["value"] = {"another", "list"};
+
+    // count elements
+    j["size"] = j.size();
+    
+    // pretty print with indent of 4 spaces
+    std::cout << std::setw(4) << j << '\n';
+}