document preferred methods for extensions

[mr-c]

What are the best practices for developing and clearly marking an extension?

[kellrott]

Currently the only extensions are backend parameters (part of developing 1.1 proposal), meant to allow user to communicate additional requirements to backend engine (for example Slurm queue preferences). Available flags are listed in the getServiceInfo packet: https://github.com/ga4gh/task-execution-schemas/blob/develop-1.1/openapi/task_execution_service.openapi.yaml#L589