add intro to info lookups section

## Getting information from other nodes

A common pattern in Puppet is to deploy resources on a given host with

information from another host. For example, you might want to grant

access to host A from host B. And while you can hardcode host B's IP

address in host A's manifest, it's not good practice: if host B's IP

address changes, you need to change the manifest, and that practice

makes it difficult to introduce host C into the pool...

So we need ways of having a node use information from other nodes in

our Puppet manifests. There are 5 methods in our Puppet source code at

the time of writing:

 * Exported resources

 * PuppetDB lookups

 * Puppet Query Languaeg (PQL)

 * LDAP lookups

 * Hiera lookups

This section walks through how each method method works, outlining the

advantage/disadvantage of each.

### Exported resources

Our Puppet configuration supports [exported resources](https://puppet.com/docs/puppet/latest/lang_exported.html), a key

same way that exported resources trust the exporter, we trust that the

node exported the right fact. So it's in theory possible that a

compromised Puppet node exports an evil IP address in the above

example, granting access to an attacker instead of the proper node.

example, granting access to an attacker instead of the proper node. If

that is a concern, consider using LDAP or Hiera lookups instead.

Also note that this will eventually fail when the node goes down:

after a while, resources are expired from the PuppetDB server and the

want to eventually revoke access to nodes that go away, but it's still

something to keep in mind.

Keep in mind that the `networking.ip` fact, in the above example,

might be incorrect in the case of a host that's behind NAT. In that

case, you should use LDAP or Hiera lookups.

Note that this could also be implemented with a `concat` exported

resource, but much harder because you would need some special case

when no resource is exported (to avoid adding the `deny`) and take