title: Exporting data in Roxygen2 so that they are available without requiring data()
body: <p>After reading questions such as <a href="https://stackoverflow.com/questions/9561684/documenting-dataset-with-roxygen2" title="this one">this SO question on documenting a data set with Roxygen</a> I have managed to document a dataset (which I will refer to as <code>cells</code>) and it now appears in the list generated by <code>data(package="mypackage")</code> and is loaded if I run the command <code>data(cells)</code>. After this, <code>cells</code> will appear when <code>ls()</code> is run.</p>
  
  <p>However, in many packages the data is immediately available without requiring a <code>data()</code> call. Also, the data names do not appear when <code>ls()</code> is run. An example is the <code>baseball</code> data set that comes with <code>plyr</code>. I have looked at the source for <code>plyr</code> and I cannot see how this is done.</p>