title: discord.py add to json file on server add
body: <p>I am wanting my bot to add the server ID and prefix of choice when they do $prefix (desired prefix) to the JSON file called settings.json. I have an example of this JSON file below.</p>
  
  <pre><code>{
    "496019377515266060": "$"
  }
  </code></pre>
  
  <p>I need it so when users type $prefix (desired prefix) it will add their server ID and the prefix of choice if not there or if it is there it will just update the prefix. I have got as far as making custom prefixes but I cannot make it so users can change them.</p>
  
  <p><strong>NOTE</strong>
  I am not using the rewrite branch.</p>