Update README.md

Added section below:

**Requirements**: an Internet connection for the initial Toolbox App installation.

1. Install the [latest version of R](http://cran.r-project.org/).
2. Launch the [Toolbox Application](http://ohi-science.org/pages/install.html).
3. Explore global results [using the Toolbox App](https://github.com/OHI-Science/ohimanual/blob/master/tutorials/using_the_ohi_toolbox_app.md#using-the-ohi-toolbox-app)