Installation problems, incorrect instructions in readme.md

The installation guild for this package is the following; however, this results in an error. 

```
Pkg.clone("https://github.com/lanl-ansi/MINLPLib.jl.git")
```

For windows, at least, it should be the following line. 

```
Pkg.add(url="https://github.com/lanl-ansi/MINLPLib.jl")
```