readme - saf validate threshold -T and -F flag

[p-oneil]

• the example usage -F output.yaml is not a descriptive filename. It should be something like threshold.yaml
• there should be a second usage example with an inline json object using -T in addition to the existing -F example
• we should consider changing the flag from -T to -J for inline json to be clearer since -T might be confused with "threshold" instead of its intended use for json objects