rework plugin description: add instructions to find settings panel
<description><![CDATA[
Add icons for files like Travis YML, Appveyor YML, etc.<br/>
+ See <i>Settings &gt; Appearance &amp; Behavior &gt; Extra Icons</i> to select extra icons to (de)activate.<br/>
<img src="https://raw.githubusercontent.com/jonathanlermitage/intellij-extra-icons-plugin/master/misc/screenshots/css_sprites.png" alt="icons preview"/>
]]>
</description>
