Complete missing info in project structure spec

[niksirbi]

The project structure specification introduced in #24 leaves out a few things which need to be added after discussion:

1. the _firstkeypoint.json label files. What should they contain, how are they named, when and where should we require them?
2. The metadata.json files. We should clarify which exact files we require and where they should be placed.

[niksirbi]

For "firstkeypoint.json", we decided that it will have the same file name and format as the cliplabels file, but:

• it will be only included in the Test split
• its name will end in _startlabels.json instead of _cliplabels.json.
• it will contain the labels for all keypoints present on the first frame of the corresponding clip.