Auto merge of #431 - jeremyjh:patch-1, r=KiChjang

Update README.md to include yasm

ICU build fails without an assembler; `brew install yasm` resolves this.

```
DEBUG: configure: error: Building ICU requires either yasm or a GNU assembler. If you do not have either of those available for this platform you must use --without-intl-api
```

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Author: bors-servo

README.md

@@ -17,7 +17,7 @@ For installation of the Rust toolchain see [`https://rustup.rs/`](https://rustup
 #### On macOS (Homebrew)
 
 ```sh
-brew install automake pkg-config python2 cmake
+brew install automake pkg-config python2 cmake yasm
 ```
 
 ## Building the project