message: Clarify migration instructions
diff: <ol>
  <li>Ensure all dependencies are installed with <code>npm install</code></li>
  <li>Start DIM development mode with <code>npm start</code></li>
  - <li>Remove this version of the unpacked DIM extension</li>
  + <li>In Chrome, remove this version of the unpacked DIM extension</li>
  <li>Add the new one from the <code>dist/</code> directory.</li>
  </ol>
  