Add instructions for deploying a NATS cluster for OpenFaaS

[welteki]

Description

Add instructions for deploying an external NATS cluster for OpenFaaS.

These instruction used to be hosted in the customers repo.

Motivation and Context

• I have raised an issue to propose this change (required)

Add instructions for production NATS deployment to the OpenFaaS docs.

How Has This Been Tested?

Verified and updated instructions for the latest NATS chart.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have signed-off my commits with git commit -s

[alexellis]

Looks ok