Write json.md content

Author: wravery

doc/json.md

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+# Converting to/from JSON
+
+## `graphqljson` Library Target
+
+Converting between `graphql::response::Value` in [GraphQLResponse.h](../include/GraphQLResponse.h)
+and JSON strings is done in an optional library target called `graphqljson`.
+
+## Default RapidJSON Implementation
+
+The included implementation uses [RapidJSON](https://github.com/Tencent/rapidjson)
+release 1.1.0, but if you don't need JSON support, or you want to integrate
+a different JSON library, you can set `GRAPHQL_USE_RAPIDJSON=OFF` in your
+CMake configuration.
+
+## Using Custom JSON Libraries
+
+If you want to use a different JSON library, you can add implementations of
+the functions in [JSONResponse.h](../include/JSONResponse.h):
+```cpp
+namespace graphql::response {
+
+std::string toJSON(Value&& response);
+
+Value parseJSON(const std::string& json);
+
+} /* namespace graphql::response */
+```
+
+You will also need to update the [CMakeLists.txt](../src/CMakeLists.txt) file
+in the [../src](../src) directory to add your own implementation. See the
+comment in that file for more information:
+```cmake
+# RapidJSON is the only option for JSON serialization used in this project, but if you want
+# to use another JSON library you can implement an alternate version of the functions in
+# JSONResponse.cpp to serialize to and from GraphQLResponse and build graphqljson from that.
+# You will also need to define how to build the graphqljson library target with your
+# implementation, and you should set BUILD_GRAPHQLJSON so that the test dependencies know
+# about your version of graphqljson.
+option(GRAPHQL_USE_RAPIDJSON "Use RapidJSON for JSON serialization." ON)
+```