doc: add reference to full email address and purpose for lists

Added purpose for lists to the mailing list table. Also added headers.

Some grammar fixups. Promote WebChat IRC interface over client
interface as many people don't have clients installed anymore 8-(.

Added a sentence that guides the user to the address of the mailing
list.  I added this after getting an issue about a request that should
have gone to the mailing list. I realised there was no reference to
the mailing lists' domain: @lists.sourceforge.net on the contact page.

I think the linked subscribe page used to provide a hint, but it
doesn't look like it does anymore.

I wonder if we should add a mailto: link there? Does sphinx obscure it
enough to prevent bot spamming?

Author: rouilj

website/www/contact.txt

@@ -12,17 +12,22 @@ We maintain the following mailing lists:
 
 .. rst-class:: linkspacing
 
-=================== ==========================
-roundup-users_      archives `[SourceForge]`__
-roundup-devel_      archives `[SourceForge]`__
-roundup-checkins_   archives `[SourceForge]`__
-=================== ==========================
+=================== ========================== ===============================
+List Name/Signup    Archive link               Purpose
+=================== ========================== ===============================
+roundup-users_      archives `[SourceForge]`__ how to use Roundup
+roundup-devel_      archives `[SourceForge]`__ develop new Roundup releases
+roundup-checkins_   archives `[SourceForge]`__ automated check-in messages
+=================== ========================== ===============================
+
+These lists are open-access, but moderated to protect them against spam.
+The email address is
+``<the list name from above>@lists.sourceforge.net``.
 
-These lists are moderated to protect them against spam. 
 Please sign up if you intend to mail repeatedly.
 
-You may talk to Roundup developers directly via IRC irc://irc.oftc.net/roundup
-using IRC client or through WebChat_ interface.
+You may talk to Roundup developers directly using the `WebChat IRC`_
+interface. You can also use an IRC client: irc://irc.oftc.net/roundup.
 
 
 .. _roundup-users: https://sourceforge.net/projects/roundup/lists/roundup-users
@@ -35,4 +40,4 @@ using IRC client or through WebChat_ interface.
 
 .. __: https://sourceforge.net/p/roundup/mailman/roundup-checkins/
 
-.. _WebChat: https://webchat.oftc.net/?randomnick=1&channels=roundup&prompt=1
+.. _WebChat IRC: https://webchat.oftc.net/?randomnick=1&channels=roundup&prompt=1