add 'smtp-tls-checker'

- signed-off-by: trimstray <trimstray@gmail.com>

README.md

@@ -507,6 +507,7 @@ Only main chapters:
 ##### :black_small_square: Mail
 
 <p>
+&nbsp;&nbsp;:small_orange_diamond: <a href="https://luxsci.com/smtp-tls-checker"><b>smtp-tls-checker</b></a> - check an email domain for SMTP TLS support.<br>
 &nbsp;&nbsp;:small_orange_diamond: <a href="https://mxtoolbox.com/SuperTool.aspx"><b>MX Toolbox</b></a> - all of your MX record, DNS, blacklist and SMTP diagnostics in one integrated tool.<br>
 &nbsp;&nbsp;:small_orange_diamond: <a href="http://www.blacklistalert.org/"><b>blacklistalert</b></a> - checks to see if your domain is on a Real Time Spam Blacklist.<br>
 &nbsp;&nbsp;:small_orange_diamond: <a href="http://multirbl.valli.org/"><b>MultiRBL</b></a> - complete IP check for sending Mailservers.<br>