Add some instructions on testing your additions as a contributor

[ryanpitts]

Pulling an idea from @auremoser in this pull request into a separate issue:

I haven't seen good advice about particular read the docs contributors beyond their own installation instructions but something in the readme that defines the ideal process would be cool too, like:

* Fork the repo to your own Github account
* Make your changes
    * (if in the browser) edit the files you'd like to update and save/commit changes
    * (if on the command line) clone your fork of the repository, commit your changes
* Create a Pull Request from your fork to the master branch of the Field Guide to Open Source in the Newsroom repo
* Describe your changes in the Pull Request window and why you made them
    * link to any issues you're responding to in the queue so they can be closed
    * select a reviewer if you'd like, perhaps the person who authored the issue you're responding to.
* Submit your Pull Request, someone will attend to it shortly!
* Well done! You're an open source contributor!