clarify NNVM’s LLVM requirement (#2117)

docs/install/from_source.rst

@@ -42,7 +42,8 @@ The minimal building requirements are
 - A recent c++ compiler supporting C++ 11 (g++-4.8 or higher)
 - CMake 3.5 or higher
 - We highly recommend to build with LLVM to enable all the features.
-- It is possible to build without llvm dependency if we only want to use CUDA/OpenCL
+- It is possible to build TVM without the LLVM dependency if we only want to use CUDA/OpenCL
+- If we want to use the NNVM compiler, then LLVM is required
 
 We use cmake to build the library.
 The configuration of tvm can be modified by `config.cmake`.