[Suggestion] Add the format of the extrinsics data with README.md

[Ianpengg]

Can you add the format of the extrinsics data with README.md in the extrinsics folder?
After downloaded the data I'm confused about what does the each number means in extrinsic file
Finally I found that how to use the extrinsic data in tranform.py

Each parameters in extrinsic file represent (x, y, z, r, p, y)