mKernel

Merge pull request #25 from uccl-project/yzhou/add_slack

[README] add slack channel

[Multinode] Fix int overflow for gemm_rs kernel (#24)

[Multinode] Fix ForwardNotify offset width (#23)

[Multinode] Widen byte-offset locals to u64 in gemm_rs/gemm_ar/ag_gemm (#22)