Add README.md, update a bunch of comments

README.md

+# postfix-nullclient-formula
+
+This is a simple collection of states that replace default exim4 with a postfix installation that acts as a nullclient.
+Throw it onto your machine and it will submit mails easily.
+
+Two things are configurable using pillar:
+
+* relayhost (which server to use for submitting emails)
+* rootmail-destination (which address to forward mail for local users to)
+
+# pillar example
+
+```
+postfix:
+  nullclient:
+    relayhost: 'smtp.hs-hannover.de'
+    rootmail-destination: 'web-rootmails@hs-hannover.de'
+```

pillar.example

+postfix:
+  nullclient:
+    relayhost: 'smtp.example.com'
+    rootmail-destination: 'rootmails@example.com'

postfix/nullclient/etc/postfix/main.cf

+### THIS FILE IS MANAGED BY SALT STATE `postfix.nullclient`
+### YOUR CHANGES WILL BE OVERWRITTEN!
 {% set minion_id = grains['id'] %}
 # Set compatibility level
 compatibility_level = 2
 
 myorigin = /etc/mailname
-smtpd_banner = $myhostname ESMTP $mail_name (Debian/GNU)
+smtpd_banner = $myhostname ESMTP $mail_name
 biff = no
 
 # appending .domain is the MUA's job.
@@ -26,7 +28,7 @@ smtp_tls_session_cache_database = btree:${data_directory}/smtp_scache
 
 myhostname = {{ minion_id }}
 
-# Forward everything to the OTRS queue
+# Forward emails for local users to this address for easy collection
 alias_maps = static:web-rootmails@hs-hannover.de
 alias_database =
 local_recipient_maps = unix:passwd.byname