Add empty header from nlohmann/json adapter

Based on json11_adapter.hpp

include/valijson/adapters/nlohmann_json_adapter.hpp

@@ -0,0 +1,30 @@
+/**
+ * @file
+ *
+ * @brief   Adapter implementation for the nlohmann json parser library.
+ *
+ * Include this file in your program to enable support for nlohmann json.
+ *
+ * This file defines the following classes (not in this order):
+ *  - NlohmannJsonAdapter
+ *  - NlohmannJsonArray
+ *  - NlohmannJsonValueIterator
+ *  - NlohmannJsonFrozenValue
+ *  - NlohmannJsonObject
+ *  - NlohmannJsonObjectMember
+ *  - NlohmannJsonObjectMemberIterator
+ *  - NlohmannJsonValue
+ *
+ * Due to the dependencies that exist between these classes, the ordering of
+ * class declarations and definitions may be a bit confusing. The best place to
+ * start is NlohmannJsonAdapter. This class definition is actually very small,
+ * since most of the functionality is inherited from the BasicAdapter class.
+ * Most of the classes in this file are provided as template arguments to the
+ * inherited BasicAdapter class.
+ */
+
+#pragma once
+#ifndef __VALIJSON_ADAPTERS_NLOHMANN_JSON_ADAPTER_HPP
+#define __VALIJSON_ADAPTERS_NLOHMANN_JSON_ADAPTER_HPP
+
+#endif