(SERVER-138) Created puppet.conf differences doc

This commit introduces a new document which contains some information
about the differences in supported settings from the `puppet.conf` file
for Puppet Server as opposed to the Ruby-based Puppet master.
@nfagerlund and @ahpook - @cprice404 raised some concerns that documenting Puppet Server's current differences in support for the `configtimeout`, `http_proxy_host`, and `http_proxy_port` settings vs. a Ruby master might be more confusing than helpful for users.  For example, users may misinterpret this to mean that puppet agents would be dropping support for these settings when the differences only pertain to a Puppet Server master making outbound HTTP client requests.  I'd appreciate your opinions on how/if we should document these particular settings.

fyi - We also have a JIRA ticket which covers the lack of support in Puppet Server for these settings - https://tickets.puppetlabs.com/browse/SERVER-156.