Add instructions on how to install the JSON library (#159)

Author: jonas

docs/src/paradox/obtaining-bindgen/cmake.md

@@ -3,15 +3,18 @@
 Building `scala-native-bindgen` requires the following tools and libraries:
 
  - [CMake] version 3.9 or higher
+ - [GNU Make]
  - [LLVM] and [Clang] version 5.0 or 6.0.
  - [nlohmann/json] version 3.1.2 or higher
 
 ```sh
 # On apt-based systems
-apt install cmake clang-$LLVM_VERSION libclang-$LLVM_VERSION-dev llvm-$LLVM_VERSION-dev
+apt install cmake make clang-6.0 libclang-6.0-dev llvm-6.0-dev \
+            nlohmann-json-dev
 
 # With `brew`
-brew install cmake llvm@6
+brew tap nlohmann/json
+brew install cmake llvm@6 nlohmann_json
 ```
 
 To run the tests you also need the required Scala Native libraries.
@@ -50,6 +53,7 @@ sbt test
 ```
 
  [CMake]: https://cmake.org/
+ [GNU Make]: https://www.gnu.org/software/make/
  [LLVM]: https://llvm.org/
  [Clang]: https://clang.llvm.org/
  [Scala Native setup guide]: http://www.scala-native.org/en/latest/user/setup.html