Add changelog and doc

CHANGELOG.md

@@ -3,6 +3,7 @@
 ## Current develop
 
 ### Added (new features/APIs/variables/...)
+- [[PR 1210]](https://github.com/parthenon-hpc-lab/parthenon/pull/1210) Add cycle based output
 - [[PR 1103]](https://github.com/parthenon-hpc-lab/parthenon/pull/1103) Add sparsity to vector wave equation test
 - [[PR 1185]](https://github.com/parthenon-hpc-lab/parthenon/pull/1185) Bugfix to particle defragmentation
 - [[PR 1184]](https://github.com/parthenon-hpc-lab/parthenon/pull/1184) Fix swarm block neighbor indexing in 1D, 2D

doc/sphinx/src/outputs.rst

@@ -6,10 +6,20 @@ Outputs
 Outputs from Parthenon are controlled via ``<parthenon/output*>`` blocks,
 where ``*`` should be replaced by a unique integer for each block.
 
+The frequency of outputs can be controlled for each block separately
+and can be triggered by either (simulation) time or cycle, i.e.,
+
+- ``dt = 0.1`` means that the output for the block is written every 0.1
+  in simulation time.
+- ``dt = 100`` means that the output for the block is written every 100
+  cycles.
+
+Note that only one option can be chosen for a given block.
 To disable an output block without removing it from the input file set
-the block's ``dt < 0.0``.
+the block's ``dt < 0.0`` and ``dn < 0`` (which is also happening by default
+if the paramter is not provided in the input file).
 
-In addition to time base outputs, two additional options to trigger
+In addition to time or cycle based outputs, two additional options to trigger
 outputs (applies to HDF5, restart and histogram outputs) exist.
 
 -  Signaling: If ``Parthenon`` catches a signal, e.g., ``SIGALRM`` which