Add extra troubleshooting points, minor readability changes

documentation changes

Make README clearer about PTR record

Fixed Tiny Grammar Error

Literally added one letter

Minor change for dovecot.conf to work on v2.3

According to the wiki (https://wiki.dovecot.org/SSL/DovecotConfiguration):

"From version 2.3, you must specify path to DH parameters file using:
ssl_dh=</path/to/dh.pem"

This actually threw an error something along the lines of: "imap-login: Error: Failed to initialize SSL"

remove smtp_tls_CAfile from postfix main.cf

Change Dovecot/Postfix definition

Technically Postfix is responsible for sending *and* receiving mail between *your server and other SMTP servers* (Mail Transfer Agents). Dovecot is only so you can view your mail in email clients on your computer instead of server, which is not really *receiving*. You can have a perfectly functional email system with no IMAP/POP server at all, you'd just have to read your email over SSH or whatever

Added instructions for setting up reverse DNS

Added instructions for setting up reverse DNS

Updated README - typos

GPLv3 proposition

I was contacted by Mr. Stallman claiming he was utterly offended by this repository not being licensed under GPLv3!

Added relaxed option for DKIM.

Create user vmail, if not present.
Add 'login' option in dovecot to be able to send emails fro mutt.

Fix TLS encrypted emails to Gmail

add SPF record generation

Make sure the opendkim tools are installed

The main command for generating DKIM keys (`opendkim`) wasn't installed with just the `opendkim` package.  Added a fix so it would be installed should it not be there the first time `apt` is ran.