Merge pull request #240 from OrenLeung/patch-1

doc: add all2all factor

[ROCm/rccl-tests commit: 3a3f790efd]

projects/rccl-tests/doc/PERFORMANCE.md

@@ -140,5 +140,6 @@ To obtain a bus bandwidth which should be independent of the number of ranks _n_
 * AllGather : (_n_-1)/_n_
 * Broadcast : 1
 * Reduce : 1
+* AlltoAll: (_n_-1)/_n_
 
 The bus bandwidth should reflect the speed of the hardware bottleneck : NVLink, PCI, QPI, or network.