This is a simple fix, but comes with our first "dot tip".

Testing the Advanced CSS Class
Include the following in your template:

<p><?php block_field( 'className' );?></p>
<p><?php echo block_value( 'className' ); ?></p>

Testing the DotTip

1. You've probably disabled tips. It's easy to do. There's an [x] button included with all tips (including the default ones that show when you first use Gutenberg). Pressing this disables all tips permanently.

To re-enable tips you've got to clear your browser cookies, or even easier, type the following into your console:

wp.data.dispatch( 'core/nux' ).enableTips();

This time clear the default tips by pressing "See next tip" > "See next tip" > "See next tip" > "Got it".

2. Add a block. It's got to be a block that you have authored – we don't show the tip on blocks that you haven't authored, so that it doesn't show up for non-block-lab blocks, and it doesn't show for editors / clients who are using your blocks.

3. Click the block to show the edit interface, then in the sidebar, open the Advanced pane. You should see something like this:

Fixes #253.

Docs needed (@RobStino?)