In the section "What data needs to be kept if transferring/backing up
Dendrite?" of the FAQ, add jetstream directory to the list.
It seems to be a common mistake when moving dendrite to a different
computer, that the jetstream directory is not copied.
### Pull Request Checklist
<!-- Please read
https://matrix-org.github.io/dendrite/development/contributing before
submitting your pull request -->
* [ ] I have added Go unit tests or [Complement integration
tests](https://github.com/matrix-org/complement) for this PR _or_ I have
justified why this PR doesn't need tests
* [x] Pull request includes a [sign off below using a legally
identifiable
name](https://matrix-org.github.io/dendrite/development/contributing#sign-off)
_or_ I have already signed off privately
Signed-off-by: `Anton Molyboha <anton.molyboha@gmail.com>`
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