Add info about Equinix Metal Reserved hardware usage in the main Readme file.

[enkelprifti98]

User Story

As a user I would like to see info about Equinix Metal reserved hardware usage with clusterAPI in the main readme description.

Detailed Description

Currently info about reserved hardware usage is located here:

https://github.com/kubernetes-sigs/cluster-api-provider-packet/blob/master/docs/concepts/machine.md#reserved-instances

I would like to see that info in the main readme file along with the addition of info about passing next-available to the hardwarereservationID field.

Anything else you would like to add:

Not that I can think of.

/kind feature

[fejta-bot]

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/lifecycle stale

[fejta-bot]

Stale issues rot after 30d of inactivity.
Mark the issue as fresh with /remove-lifecycle rotten.
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/lifecycle rotten

[detiber]

/lifecycle frozen

[cprivitere]

It's worth noting the functionality has changed a lot. Machines need to be owned by a cluster now, which makes using the PacketMachine object to do this nearly unmanageable. However, PacketMachineTemplate now supports the reservation IDs, so I'm adding documentation about how you should edit the packetmachinetemplate that clusterctl generates for you.

[cprivitere]

Fixed in #409