📝 added documentation

doc/examples/contains.cpp

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+#include <iostream>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    // create some JSON values
+    json j_object = R"( {"key": "value"} )"_json;
+    json j_array = R"( [1, 2, 3] )"_json;
+
+    // call contains
+    std::cout << std::boolalpha <<
+              "j_object contains 'key': " << j_object.contains("key") << '\n' <<
+              "j_object contains 'another': " << j_object.contains("another") << '\n' <<
+              "j_array contains 'key': " << j_array.contains("key") << std::endl;
+}