Missing documentation section for `LoopOffset`

[uncenter]

Is your feature request related to a problem? Please describe. The PR for LoopOffset was merged last year and the documentation was never updated to include it (https://github.com/charmbracelet/vhs/pull/113#issuecomment-1299306531).

Describe the solution you'd like. A section in the README and an associated GIF example. I can write the quick description and code example but I'm not sure how I should go about adding a demo GIF!

---

Set Loop Offset

Set the loop offset of the recording with the Set LoopOffset command.

# Take the first 10% of the video and place it after the end.
Set LoopOffset 10%

# Take the first 10 frames and place them at the end.
# The first frame of the video will be the 11th frame.
Set LoopOffset 10

---

(example code usage is from https://github.com/charmbracelet/vhs/issues/8#issuecomment-1279619443)