added screenshot

Author: eschallock

source/User_Guide/Marketing_Campaigns/Campaigns/index.html

@@ -215,6 +215,8 @@
 Once you’ve selected the campaigns that you want to include in your CSV, click this button and select Export CSV.
 </p>
 
+<img src="{{root_url}}/images/export_multiple_campaign_stats_1.png" alt="Export CSV" />
+
 <ul>
   <li>Campaigns are ordered from most to least recent.</li>
   <li>The CSV will include a row for each day that SendGrid has gathered statistics.</li>