Many of the examples provided within documentation are using a single
dash for the command line arguments, but the telegraf executable
explicitly has two dashes.

There are also some inconsistencies with the ordering of the command
line argument examples.  I've ordered them so that the examples will
show: config, config-directory, input-filter, test

During issue #2215 it was highlighted that the current behavior where
rules without a comment are ignored is confusing for several users.

This commit improves the documentation and adds a NOTE to the sample
config to clarify the behavior for new users.

* Update README.md

* Add lock support to the IPtables input plugin

* Update iptables.go

Doc cleaning

The iptables plugin aims at monitoring bytes and packet counters
matching a given set of iptables rules.

Typically the user would set a dedicated monitoring chain into a given
iptables table, and add the rules to monitor to this chain. The plugin
will allow to focus on the counters for this particular table/chain.

closes #1471