Document how to orient buttons in a column

[iamakulov]

We received a letter asking how to make the buttons oriented in a column instead of a row. This can be done by placing <br />s between the buttons:

<div class="likely">
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</div>

This should be documented in README.

[vitkarpov]

Also, maybe it could be done using by adding likely-vertical class? It's more convenient to change the orientation in runtime (for instance, when the screen's becoming narrow)

[iamakulov]

Good point. Will discuss this with @ilyabirman.

[NikolayRys]

Gonna stick with <br/> so far.