Merge pull request #925 from zerodefect/improvement/improve_readme_js…

…on_fwd

Howto installation of json_fwd.hpp (fixes #923)

README.md

@@ -65,7 +65,7 @@ using json = nlohmann::json;
 
 to the files you want to use JSON objects. That's it. Do not forget to set the necessary switches to enable C++11 (e.g., `-std=c++11` for GCC and Clang).
 
-You can further use file [`develop/json_fwd.hpp`](https://github.com/nlohmann/json/blob/develop/develop/json_fwd.hpp) for forward-declarations.
+You can further use file [`develop/json_fwd.hpp`](https://github.com/nlohmann/json/blob/develop/develop/json_fwd.hpp) for forward-declarations. The installation of json_fwd.hpp (as part of cmake's install step), can be achieved by setting `-DJSON_MultipleHeaders=ON`:
 
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