release-notes/v5.6.0.md: include release notes for
#### <sub><sup><a name="v560-note-4348" href="#v560-note-4348">:link:</a></sup></sub> fix
* The [fly `set-team`](https://concourse-ci.org/managing-teams.html#fly-set-team) documentation when running `--help` previously suggested that a list is a valid input to any auth configuration flags. This, however, was not the case where it actually only accepts a single value per flag and if you want to configure multiple, you need to pass them individually using separate flags. The [fly `set-team`](https://concourse-ci.org/managing-teams.html#fly-set-team) help documentation will now clearly describe that each flag only accepts a single value, thanks to @niall-byrne! [#4348](https://github.com/concourse/concourse/issues/4348)
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+#### <sub><sup><a name="v560-note-4442" href="#v560-note-4442">:link:</a></sup></sub> fix
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+* `/opt/resource/out` scripts in resources could previously crash `atc` instances by outputing `null` to `stdout`, causing a nil pointer dereference. The `atc` now checks for a nil value and, if one is found, returns before dereferencing the pointer. [#4442](https://github.com/concourse/concourse/pull/4442)
