Update README.md

Otherwise I have 

```
[warn] 	::::::::::::::::::::::::::::::::::::::::::::::
[warn] 	::          UNRESOLVED DEPENDENCIES         ::
[warn] 	::::::::::::::::::::::::::::::::::::::::::::::
[warn] 	:: org.scala-lang.modules.scalajs#scalajs-tools_2.10;0.5.0-SNAPSHOT: not found
[warn] 	::::::::::::::::::::::::::::::::::::::::::::::
```