Add new command-line arguments

README.md

@@ -15,9 +15,10 @@ To use the program `swiftmpistepsim` you need to select the step of interest
 concatenate all the logs for that step into a single file. You can then
 run using:
 ```
-   mpirun -np <nranks> swiftmpistepsim <step-log>
+   mpirun -np <nranks> swiftmpistepsim <step-log> <output-log>
 ```
-which will output timings for the various MPI calls.
+which will output timings for the various MPI calls and record a log
+for the reproduction in the file `<output-log>`.
 
 To simulate SWIFT we use three threads, which run simultaneously, one that
 injects the MPI commands, i.e. initiates the interaction using calls to