Add description of webhook for registry updates

When creating a new registry repository to publish spec assets, there needs to be a webhook configured in the repo settings - with a Secret validating the hook - and a corresponding change to config files on the registry.khronos.org side. It is probably best to direct people to contact @KhronosWebservices to assist with this.

Review the repository structure and existing markup examples to find where repository setup guidance belongs. Document the webhook, its validating Secret, the corresponding registry.khronos.org configuration change, and contacting @KhronosWebservices for assistance; the work is done when these steps are clear to someone creating a registry repository.

Written by the indexing model from the issue text.