[DOCS] Add some notes about LLVM (#373)

Add some notes about LLVM before building TVM

docs/how_to/install.md

@@ -57,7 +57,7 @@ The configuration of tvm can be modified by ```config.mk```
 - First copy ```make/config.mk``` to the project root, on which
   any local modification will be ignored by git, then modify the according flags.
 - TVM optionally depends on LLVM. LLVM is required for CPU codegen that needs LLVM.
-  - LLVM 4.0 or higher is needed for build with LLVM
+  - LLVM 4.0 or higher is needed for build with LLVM. The verison of LLVM from apt may lower than 4.0. you can download pre-built version of LLVM 4.0 from [LLVM Download Page](http://releases.llvm.org/download.html#4.0.1) and choose your matched pre-built binary package using `gcc -v` in command line to check `Target` value.
   - By default CUDA and OpenCL code generator do not require llvm.
 
 ## Python Package Installation