README note about LLVM versions

fixes JuliaLang#9341

README.md

@@ -267,7 +267,7 @@ Building Julia requires that the following software be installed:
 
 Julia uses the following external libraries, which are automatically downloaded (or in a few cases, included in the Julia source repository) and then compiled from source the first time you run `make`:
 
-- **[LLVM]** (>= 3.3) — compiler infrastructure.
+- **[LLVM]** (3.3)  — compiler infrastructure. (3.4 not supported; 3.5+ mostly supported, [with caveats](https://github.com/JuliaLang/julia/issues/9336)) 
 - **[FemtoLisp]** — packaged with Julia source, and used to implement the compiler front-end.
 - **[libuv]** — portable, high-performance event-based I/O library
 - **[OpenLibm]** — a portable libm library containing elementary math functions.