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.

Use spamassassin on port 25 only

Typos + other tiny corrections, is it "honestly have" or "honestly don't have" preference towards vultr? Tried to guess...

close quotation marks in opendkim socket config

opendkim ignores the invalid SOCKET config line, causing smtpd mail send to fail.

$ sudo systemctl status opendkim
systemd[1]: Ignoring invalid environment assignment 'SOCKET=inet:8891@localhost
systemd[1]: ': /etc/sysconfig/opendkim

$ journalctl
postfix/submission/smtpd[17646]: fatal: host/service localhost/8891 not found: Device or resource busy

prevent false detection of opendkim KeyTable config

default /etc/opendkim.conf has KeyTable appear in comment doco and example.
Shouldn't trigger the grep that checks whether this config has been applied.

Fix typo in the deleting of `-o` flags

Deleting all settings starting with o seems rather arbitrary, but I don't know the default configuration on Vultr, so could be warranted :) But I'll be bold and guess the `sed` command and the following lines are supposed to be related?