Missing documentation section for `LoopOffset`

**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.

```elixir
# 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)

Start in the README and review the existing loop-offset.gif example URL referenced in the issue. Add the LoopOffset command description and code examples, include the associated demo GIF, and verify that the README renders the new section correctly.

Written by the indexing model from the issue text.