A simple plugin starter template to add extra info to posts excerpt previews, after the excerpt-content.

WordPress-Plugin-Template-Extra-Post-Info.php

<?php
/*
Plugin name: WordPress Plugin Template - Extra Post Info
Plugin URI: http://veitchdigital.com/
Description: A simple plugin starter template to add extra info to posts excerpt previews, after the excerpt-content. 
Author: Ryan Veitch
Author http://veitchdigital.com/
Version: 0.1
*/

// Call extra_post_info_menu function to load plugin menu in dashboard
add_action( 'admin_menu', 'extra_post_info_menu' );

// Create WordPress admin menu
function extra_post_info_menu(){

  $page_title = 'WordPress Extra Post Info';
  $menu_title = 'Extra Post Info';
  $capability = 'manage_options';
  $menu_slug  = 'extra-post-info';
  $function   = 'extra_post_info_page';
  $icon_url   = 'dashicons-info';
  $position   = 4.01; // Use decimal places to avoid position conflicts

  add_menu_page( $page_title,
                 $menu_title,
                 $capability,
                 $menu_slug,
                 $function,
                 $icon_url,
                 $position );

  // Call update_extra_post_info function to update database
  add_action( 'admin_init', 'update_extra_post_info' );

}

// Create function to register plugin settings in the database
function update_extra_post_info() {
  register_setting( 'extra-post-info-settings', 'extra_post_info' );
}

// Create Dashboard plugin page
function extra_post_info_page(){
?>
  <h1>Extra Post Info Settings</h1>
  <form method="post" action="options.php">
    <?php settings_fields( 'extra-post-info-settings' ); ?>
    <?php do_settings_sections( 'extra-post-info-settings' ); ?>
    <table class="form-table">
      <tr valign="top">
      <th scope="row">Extra post info:</th>
      <td><input type="text" name="extra_post_info" value="<?php echo get_option('extra_post_info'); ?>"/></td>
      </tr>
    </table>
  <?php submit_button(); ?>
  </form>
<?php
}


// Plugin logic for adding extra info to posts
if( !function_exists("extra_post_info") )
{
  function extra_post_info($content)
  {
    $extra_info = get_option('extra_post_info');
    return $content . $extra_info;
  }

// Apply the extra_post_info function on our content  
add_filter('the_content', 'extra_post_info');
}

?>