docs: add EU legal notice

features that you need to understand in order to add In-app 

Billing features into your application.</p>

<p class="note"><b>Note</b>: Ensure that you comply with applicable laws in the countries where you

distribute apps. For example, in EU countries, laws based on the

<a href="http://ec.europa.eu/justice/consumer-marketing/unfair-trade/unfair-practices/">Unfair

Commercial Practices Directive</a> prohibit direct exhortations to children to buy advertised

products or to persuade their parents or other adults to buy advertised products for them.

See the

<a href="http://ec.europa.eu/consumers/enforcement/docs/common_position_on_online_games_en.pdf">position

of the EU consumer protection authorities</a> for more information on this and other topics.

</p>

<h2 id="api">In-app Billing API</h2>

<p>Your application accesses the In-app Billing service using an API that is 

exposed by the Google Play app that is installed on the device. The Google Play