README: Missing link detailing supported annotation formats

[daavoo]

There are multiple mentions to annotations both in the README and the Get started (which appears to be misleading #5 ) but no direct link explaining what are the supported annotation formats.

The mentions of the supported annotations format I have found are in the INDEX cmd ref and in the last section of Get Started

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Side note: The intro in the INDEX cmd ref says:

annotations in default format (one .json file per every data object sharing the object name)

However, according to the extended explanation of the flags, it seems that strict-pairs, annotation-only, tensorflow-inferred formats can all be detected by default.