Document release process.

Fixes #292.

This is mostly an initial sketch for discussion.

This also at least partially addresses #86, #87, #126, #277.
Why this change? Both paths seem to exist and are actually symlinks to `xtables-multi`

```
[~]$ ls -l /sbin/iptables
lrwxrwxrwx. 1 root root 13 Apr 12 15:08 /sbin/iptables -> xtables-multi
[~]$ ls -l /sbin/xtables-multi 
-rwxr-xr-x. 1 root root 92504 Jul 31  2013 /sbin/xtables-multi
[~]$ ls -l /usr/sbin/xtables-multi 
-rwxr-xr-x. 1 root root 92504 Jul 31  2013 /usr/sbin/xtables-multi
```

 * http://manpages.ubuntu.com/manpages/trusty/man8/xtables-multi.8.html