Add a note about keyword filtering in Docs / Email

templates/docs/email.html

@@ -2,6 +2,12 @@
 <p>As an alternative to HTTP/HTTPS requests, you can "ping" checks by
 sending email messages to special email addresses.</p>
 <p><img alt="Email address for pinging via email" src="IMG_URL/emails.png" /></p>
+<p>By default, SITE_NAME will consider any email received at the displayed address as
+a "success" signal. You can also configure SITE_NAME to look for specific
+keywords in message's subject line to decide if the message is "success"
+or a "failure" signal. You can set up the keywords in the
+<strong>Filtering Rules</strong> dialog:</p>
+<p><img alt="Setting filtering rules" src="IMG_URL/filtering_rules.png" /></p>
 <h2>Use Case: Newsletter Delivery Monitoring</h2>
 <p>Consider a cron job that runs weekly and sends weekly newsletters
 to a list of email addresses. You have already set up a check to get alerted

templates/docs/email.md

@@ -5,6 +5,14 @@ sending email messages to special email addresses.
 
 ![Email address for pinging via email](IMG_URL/emails.png)
 
+By default, SITE_NAME will consider any email received at the displayed address as
+a "success" signal. You can also configure SITE_NAME to look for specific
+keywords in message's subject line to decide if the message is "success"
+or a "failure" signal. You can set up the keywords in the
+**Filtering Rules** dialog:
+
+![Setting filtering rules](IMG_URL/filtering_rules.png)
+
 ## Use Case: Newsletter Delivery Monitoring
 
 Consider a cron job that runs weekly and sends weekly newsletters