Update CONTRIBUTING.md to provide a more involved example of creating a command

How do you create your own slash command? How do you get the slash command to do more than just open a new tab which is what is shown in the example given in the documentation [here](https://github.com/jessepollak/command/blob/master/CONTRIBUTING.md).

I would like to create a `/google` command that would search the web with google and get the url and title of the search result the user selected and insert it into the textarea.