Add description of testdata

testdata/README

+There are two example SWIFT mpi logging output logs. A small one for checking
+program developments and a larger one that exhibits the unwanted behaviour on
+the COSMA7 cluster.
+
+The small dataset: `EAGLE_6-mpiuse_report-step2-4ranks-small-step.dat`, is the
+second step of a 4 rank run of the EAGLE_6 low z example.
+
+The larger dataset: `EAGLE_25-mpiuse_report-step18302-16ranks-inter-step.dat`,
+uses the high z EAGLE_25 with 16 ranks. Decompresses to 21M.