moved documentation to index markdown file

doc/index.md

@@ -0,0 +1,21 @@
+# JSON for Modern C++
+
+These pages contain the API documentation of JSON for Modern C++, a C++11 header-only JSON class.
+
+- @link nlohmann::basic_json `basic_json` class @endlink
+- Types
+  - @link nlohmann::basic_json::array_t arrays @endlink
+  - @link nlohmann::basic_json::object_t objects @endlink
+  - @link nlohmann::basic_json::string_t strings @endlink
+  - @link nlohmann::basic_json::boolean_t booleans @endlink
+  - numbers
+    - @link nlohmann::basic_json::number_integer_t signed integers @endlink
+    - @link nlohmann::basic_json::number_unsigned_t unsigned integers @endlink
+    - @link nlohmann::basic_json::number_float_t floating-point @endlink
+
+@copyright Copyright © 2013-2016 Niels Lohmann. The code is licensed under the [MIT License](http://opensource.org/licenses/MIT).
+
+@author [Niels Lohmann](http://nlohmann.me)
+@see https://github.com/nlohmann/json to download the source code
+
+@version 2.0.0