Document how users should bring in updates from the template

[KatieProchilo]

When the workflows are updated at https://github.com/Azure-Samples/Speech-Service-DevOps-Template, unlike forks, these changes cannot be pulled to the user's template repos.

Instead they'll have to create a branch in their template repo, manually copy in the updated files, and handle any merging issues if they've modified the YAML in any way.