Update README.md

Reply-To header works with mailmerge but there's not any documentation about it. An example is added to the advanced template.

README.md

@@ -208,6 +208,7 @@ This example will send progress reports to students.  The template uses more of
 TO: {{email}}
 SUBJECT: EECS 280 Mid-semester Progress Report
 FROM: My Self <[email protected]>
+REPLY-TO: My Reply Self <[email protected]>
 
 Dear {{name}},