introducing ENABLE_SRS env variable (Closes: #906, #852)

* making postsrsd optional
* added tests, added documentation

README.md

@@ -83,6 +83,10 @@ You're done!
 
 And don't forget to have a look at the remaining functions of the `setup.sh` script
 
+#### SPF/Forwarding Problems
+
+If you got any problems with SPF and/or forwarding mails, give [SRS](https://github.com/roehling/postsrsd/blob/master/README.md) a try. You enable SRS by setting `ENABLE_SRS=1`. See the variable description for further information.
+
 #### For informational purposes:
 
 Your config folder will be mounted in `/tmp/docker-mailserver/`. To understand how things work on boot, please have a look at [start-mailserver.sh](https://github.com/tomav/docker-mailserver/blob/master/target/start-mailserver.sh)
@@ -260,6 +264,11 @@ Configures the handling of creating mails with forged sender addresses.
   - **empty** => Mail address spoofing allowed. Any logged in user may create email messages with a forged sender address. See also [Wikipedia](https://en.wikipedia.org/wiki/Email_spoofing)(not recommended, but default for backwards compatibility reasons)
   - 1 => (recommended) Mail spoofing denied. Each user may only send with his own or his alias addresses. Addresses with [extension delimiters](http://www.postfix.org/postconf.5.html#recipient_delimiter) are not able to send messages.
 
+##### ENABLE_SRS
+Enables the Sender Rewriting Scheme. SRS is needed if your mail server acts as forwarder. See [postsrsd](https://github.com/roehling/postsrsd/blob/master/README.md#sender-rewriting-scheme-crash-course) for further explanation.
+  - **0** => Disabled
+  - 1 => Enabled
+
 ##### PERMIT_DOCKER
 
 Set different options for mynetworks option (can be overwrite in postfix-main.cf)