MacPorts Clang 3.9 Instructions

[casey]

I use MacPorts on OS X, and was able to build bindgen. You might want to consider adding instructions to the readme:

1. To install clang:
   port install clang-3.9
2. And to build rust-bindgen:

LIBCLANG_PATH=/opt/local/libexec/llvm-3.9/lib \
LD_LIBRARY_PATH=/opt/local/libexec/llvm-3.9/lib \
cargo build

[emilio]

Thanks! I added it in #65 :)

[casey]

Awesome :) You're welcome!