Very simple demonstration of adding placeholder or notice text to the customizer when you want to surface information in the customizer but don't actually need to save data

customize-notice-control.php

<?php
/**
 * Very simple demonstration of adding placeholder or notice text to the
 * customizer when you want to surface information in the customizer but don't
 * actually need to save data.
 */

/**
 * A dummy control to display placeholder text in the customizer
 *
 * @see WP_Customize_Control
 */
class Luigi_WP_Customize_Notice_Control extends WP_Customize_Control {
	/**
	* Control type
	*
	* @since 0.0.1
	*/
	public $type = 'notice';

	/**
	* HTML content to display
	*
	* @since 0.0.1
	*/
	public $content = '';

	public function render_content() {
		if ( !empty( $this->label ) ) :	?>
			<div class="customize-control-title">
				<?php echo esc_html( $this->label ); ?>
			</div>
		<?php endif; ?>
		<div class="customize-notice-content">
			<?php echo $this->content; ?>
		</div>
		<?php
	}
}

/**
 * Add a section and control
 *
 */
add_action( 'customize_register', 'luigi_customizer_add_controls' );
function luigi_customizer_add_controls( $wp_customize ) {

	// In this case, I add a section to surface the custom font features that
	// are available if they activate a specific plugin.
	if ( !is_plugin_active( 'typecase/typecase.php' ) ) {
		$wp_customize->add_section(
			'luigi_typecase_placeholder',
			array(
				'title'      => __( 'Theme Fonts', 'luigi' ),
				'priority'   => 30,
			)
		);

		$wp_customize->add_control(
			new Luigi_WP_Customize_Notice_Control(
				$wp_customize,
				'luigi_typecase_placeholder',
				array(
					'label'     => __( 'Install Typecase', 'luigi' ),
					'section'   => 'luigi_typecase_placeholder',
					'settings'   => array(),
					'content'   => sprintf(
						// Translators: 1 and 2 wrap a link to install the plugin. 3 and 4 wrap a link to documentation on this.
						__( 'Install and activate the %1$sTypecase plugin%2$s if you would like to change the fonts used on your site. %3$sLearn more%4$s', 'luigi' ),
						'<a href="' . esc_url( admin_url( 'plugin-install.php?tab=plugin-information&plugin=typecase' ) ) . '" target="_blank">',
						'</a>',
						'<a href="http://doc.themeofthecrop.com/themes/luigi/user/faq#typecase" target="_blank">',
						'</a>'
					),
				)
			)
		);
	}
}

Thanks Weston. Updated.

The same can be achieved without extending the class ...

            $wp_customize->add_control('luigi_typecase_placeholder', array(
                'label'         => __( 'Install Typecase', 'luigi' ),
                'description'   => sprintf(
						// Translators: 1 and 2 wrap a link to install the plugin. 3 and 4 wrap a link to documentation on this.
						__( 'Install and activate the %1$sTypecase plugin%2$s if you would like to change the fonts used on your site. %3$sLearn more%4$s', 'luigi' ),
						'<a href="' . esc_url( admin_url( 'plugin-install.php?tab=plugin-information&plugin=typecase' ) ) . '" target="_blank">',
						'</a>',
						'<a href="http://doc.themeofthecrop.com/themes/luigi/user/faq#typecase" target="_blank">',
						'</a>'
					),
                'section'       => 'luigi_typecase_placeholder',
                'settings'      => array(),
                'type'          => 'hidden'
            ));