Add a command line usage section to the  README file

I added a section to the README file that includes some of the instructions posted here: http://ecodataretriever.org/cli.html . However, some of the instructions on that website now appear to be out-dated and therefore it is important to document the current usage of the command line interface. Specifically this addition provides the correct example of how to use <code>retriever install</code> as the name of the dataset is no longer the first argument provided and the specification of the database format has changed. Hopefully this will be helpful to others.