Add mmm.pl

Signed-off-by: Thomas Hochstein <thh@inter.net>

README.md

@@ -13,3 +13,12 @@ to the format of the Wordpress plugin
 
 Usage: `footnotes --to mmd|wp [--file $inputfile]`
 
+### mmm.pl
+
+`mmm` (*MIME multipart/alternative from Markdown*) can
+create a MIME `multipart/alternative` mail, containing a
+`text/plain` part (in Markdowen) and a `text/html` part, from
+a Markdown file. Other mail headers (From:, To:, Subject:, ...)
+can be prepended from a template file.
+
+Usage: `mmm [--file $inputfile] [--headers $templatefile]`