title: Set state property from URL using react-router
body: <p>I have a container component with a modal in it that is opened and closed based on a state property.</p>
  
  <p>I want to control this via the URL, i.e. I want to have</p>
  
  <pre><code>/projects - the modal is NOT open
  /projects/add - the modal IS open
  </code></pre>
  
  <p>As well as being able to link directly to it, I want the URL to change when I click on links within the main container to open the modal.</p>
  
  <p>Can someone explain how I could do this, or point me in the right direction of a good tutorial?</p>