fix #200: Mention Isso as an alternate comment system.

This small change to *acrylamid* doc index page should fix [issue no. 200](https://github.com/posativ/acrylamid/issues/200), although I'm not confident in its english phrasing.

By the way, I also mention an other alternative for comment system, used by a Pelican plugin, which use mailto: link to send formatted comments for reviewing (inserting them in doc. remains to the blogger). This has the advantage to not require any additional piece of software on the server side.